Recreate labs virtualenv when its interpreter is missing

[Hashim1999164]

Changes

• Recreate the labs virtualenv with venv clear when the interpreter path is missing or dangling
• That unblocks databricks labs install after Homebrew retires the Python the venv was built against

Closes #6078

Why

python m venv leaves existing interpreter symlinks in place. After a Python upgrade the symlink still points at a deleted binary, pip install fails, and every retry hits the same leftover venv.

Tests

• go test ./cmd/labs/project covering TestShouldRecreateVenv

[github-actions]

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Trigger:
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Inputs:

• PR number: 6312
• Commit SHA: e189de082f75a9e7976642a74473ccdd7a8ea278

Checks will be approved automatically on success.

[github-actions]

Approval status: pending

/cmd/labs/ - needs approval

Files: cmd/labs/project/installer.go, cmd/labs/project/installer_venv_test.go
Eligible: @alexott, @asnare

General files (require maintainer)

Files: .nextchanges/cli/labs-venv-missing-interpreter.md
Based on git history:

• @pietern -- recent work in cmd/labs/project/, .nextchanges/cli/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}