Update devenv.nix to fix issue with gh hooks

[CristhianMotoche]

I was getting this error without this small change:

> devenv shell
✓ Validating lock 1 files                                                                                                                                                         2ms
✓ ▸ Configuring shell 2 steps                                                                                                                                                    37ms
✓ Loading tasks                                                                                                                                                                   4ms
  └ ✓ Evaluating devenv.config.task.config cached                                                                                                                                 1ms
✗ Running tasks 6/6                                                                                                                                                             369ms
  ├ ✗ devenv:enterShell failed                                                                                                                                                    0ms
  │ └ ✗ devenv:git-hooks:install failed (8 lines)                                                                                                                               246ms
  │ error: Command `get git hooks dir` exited with an error:
  │
  │ [status]
  │ exit status: 1
  │
  │ [stderr]
  │ error: tool 'git' not found
  │
  │   └ ✓ devenv:files                                                                                                                                                           71ms
  │     └ ✓ devenv:files:cleanup                                                                                                                                                 51ms
  └ ✗ devenv:enterTest failed                                                                                                                                                     0ms
    └ ✗ devenv:git-hooks:run failed                                                                                                                                               0ms
      └ ✗ devenv:git-hooks:install failed (8 lines)                                                                                                                             246ms
  │ error: Command `get git hooks dir` exited with an error:
  │
  │ [status]
  │ exit status: 1
  │
  │ [stderr]
  │ error: tool 'git' not found