[build] Share provisioned .NET SDK across worktrees

[simonrozsival]

Context

Each git worktree currently provisions the same pinned .NET SDK into its own
bin/<configuration>/dotnet directory. Developers using several worktrees
therefore download and store duplicate SDK installations.

Changes

• Treat DOTNET_INSTALL_DIR as an opt-in shared SDK base for local builds.

• Install each pinned SDK once under:

  $DOTNET_INSTALL_DIR/<sdk-version>/
  

• Share that versioned SDK between worktrees and between Debug and Release
  builds.

• Persist the resolved SDK path in each checkout under
  bin/<configuration>/dotnet-install-location.txt, allowing later Make,
  MSBuild, and dotnet-local invocations to find it without requiring the
  environment variable to remain set.

• Keep configuration-specific workload packs and build outputs under each
  checkout's existing bin/Debug or bin/Release directories.

• Ignore DOTNET_INSTALL_DIR when TF_BUILD, GITHUB_ACTIONS, or CI is set.
  CI therefore retains the existing worktree-local
  bin/<configuration>/dotnet behavior.

• Document the opt-in setup for Unix and Windows.

Example:

export DOTNET_INSTALL_DIR="$HOME/android-dotnet-sdk"
make prepare

Validation

• Completed make prepare and make all: 0 warnings, 0 errors.
• Ran make prepare from a second worktree using the same base. The installer
  reported the SDK was already installed and emitted zero download attempts.
• Verified Debug and Release resolve the same versioned SDK root.
• Verified shell, PowerShell, Make, MSBuild, and dotnet-local path handling.
• Verified absolute paths, relative paths, and paths containing spaces.
• Verified CI markers force the existing checkout-local SDK path.

[simonrozsival]

Local validation completed:

• First worktree: DOTNET_INSTALL_DIR="$HOME/android-dotnet-sdk" make prepare succeeded and installed SDK 11.0.100-rc.1.26411.119.
• First worktree: make all then succeeded with DOTNET_INSTALL_DIR unset, proving the persisted install location is used. Final result: 0 warnings, 0 errors.
• Created a second worktree on dev/simonrozsival/shared-dotnet-sdk-cache-verification.
• Second worktree: the same make prepare succeeded and reported .NET Core SDK with version '11.0.100-rc.1.26411.119' is already installed.\n- The second preparation had 0 Attempting to download messages.\n- Both worktrees persisted the same SDK root: ~/android-dotnet-sdk/11.0.100-rc.1.26411.119/Debug/.

[simonrozsival]

Updated the shared layout to use one SDK installation for both configurations:

$DOTNET_INSTALL_DIR/<sdk-version>/

Debug and Release retain separate checkout-local outputs and separate location marker files, but both markers point to the same SDK root. Shell, PowerShell, MSBuild property evaluation, and dotnet-local were validated against the unified path.

[Copilot]

Pull request overview

This PR updates the local build bootstrapping flow to optionally share the pinned .NET SDK installation across multiple git worktrees (and across Debug/Release), while keeping workload packs and build outputs checkout-local. It does this by introducing a persisted “resolved dotnet root” file per configuration and teaching the build entry points (Make/MSBuild/dotnet-local) to honor it.

Changes:

• Add an opt-in shared SDK installation mode via DOTNET_INSTALL_DIR, installing the pinned SDK under <base>/<sdk-version>/ (disabled in CI environments).
• Persist the resolved SDK location to bin/<Configuration>/dotnet-install-location.txt and teach dotnet-local (bash/cmd) and Make’s MSBuild wrapper to use it.
• Document the shared SDK setup steps for both Unix and Windows build instructions.

Show a summary per file

File	Description
eng/install-dotnet.sh	Adds shared-install behavior for the pinned SDK and writes dotnet-install-location.txt.
eng/install-dotnet.ps1	PowerShell equivalent of shared-install behavior and persisted location file.
dotnet-local.sh	Resolves dotnet root from dotnet-install-location.txt when present.
dotnet-local.cmd	Resolves dotnet root from dotnet-install-location.txt when present and factors probing into a helper label.
Documentation/building/windows/instructions.md	Documents DOTNET_INSTALL_DIR workflow for Windows and clarifies dotnet-local behavior.
Documentation/building/unix/instructions.md	Documents DOTNET_INSTALL_DIR workflow for Unix and clarifies dotnet-local behavior.
Directory.Build.props	Adds MSBuild-time resolution of DotNetPreviewPath based on DOTNET_INSTALL_DIR and/or the persisted location file.
build-tools/scripts/msbuild.mk	Updates Make/MSBuild invocation to use the persisted dotnet root file and quotes the dotnet tool path.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 2
• Review effort level: Lite