test(spanner): Fix Spanner presubmit trampoline and system tests

[sakthivelmanii]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request updates the PROJECT_ROOT calculation in trampoline_v2.sh by removing a hardcoded subdirectory suffix and adds a shallow git fetch to ensure the commit history is available for diffing in CI environments. Review feedback suggests double-quoting $(pwd) to prevent word splitting in paths containing spaces and removing the suppression of stderr in the git fetch command to improve observability for debugging CI failures.