fix(gitSetupShell): do not hardcode bash location

[xav-ie]

What / Why / How

The cross-project-attribution test sets up temp git repos through
shell: "/bin/bash". That path does not exist on NixOS, where bash lives in the
Nix profile rather than /bin, so the whole suite fails at setup with ENOENT.

Switch the hardcoded /bin/bash to a bare "bash" so Node resolves it from
PATH. This keeps bash semantics (the setup runs git init && git remote add)
and matches how shipped code already picks a shell (commandExists("bash") in
src/runtime.ts). win32 keeps execSync's default shell, unchanged.

Affected platforms

Test-only change; no adapter or product behavior changes. Impacts any host
where bash is not at /bin/bash (NixOS, and other non-FHS layouts).

• All platforms

Test plan

Existing tests/integration/cross-project-attribution.test.ts is the coverage.
On a host without /bin/bash it goes from 5 failing (ENOENT in beforeAll setup)
to 5 passing. No product code changed, so no new test was added; adding one
would only re-test Node + bash.

Verified locally:

• before (bare next): 5 failed, all ENOENT from /bin/bash
• after: 5 passed

Checklist

• Tests added/updated (the fixed suite is the coverage; see Test plan)
• npm test passes
• npm run typecheck passes
• Docs updated if needed (n/a)
• No Windows path regressions (forward slashes only)
• Targets next branch