chore(ci): enable workflows and scripts on preview

[noahdietz]

Updates branch-based workflows to also run on pull requests against preview branch, and set the TARGET_BRANCH (derived from Context variables github.base_ref for PRs, and github.event.merge_group.base_ref for merge_group) environment variable for the testing scripts to reference in their git comparison operations.

http://b/450631301

[gemini-code-assist]

Summary of Changes

Hello @noahdietz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the continuous integration setup by enabling workflows to run against the preview branch and introducing a dynamic TARGET_BRANCH variable. These changes improve the flexibility and accuracy of CI checks, ensuring that testing scripts correctly identify the base branch for comparison operations, thereby streamlining the development and merging process for features targeting different environments.

Highlights

• CI Workflow Expansion: Branch-based workflows are now configured to also execute on pull requests targeting the preview branch, extending CI coverage.
• Dynamic Target Branching: A new TARGET_BRANCH environment variable is introduced, derived from GitHub context variables (github.base_ref for PRs and github.event.merge_group.base_ref for merge groups), allowing CI scripts to dynamically reference the correct base branch for Git operations.
• Script Modernization: The ci/run_conditional_tests.sh script has been updated to utilize the new TARGET_BRANCH variable, replacing hardcoded references to main for git diff and git fetch commands.

Changelog

• ci/run_conditional_tests.sh
  • Introduced TARGET_BRANCH variable, defaulting to main if not set.
  • Modified GIT_DIFF_ARG for 'presubmit' builds to use origin/$TARGET_BRANCH... instead of origin/main....
  • Updated git fetch commands for both 'presubmit' and 'continuous' builds to fetch from origin $TARGET_BRANCH.

Ignored Files

• Ignored by pattern: .github/workflows/** (3)
  • .github/workflows/docs.yml
  • .github/workflows/lint.yml
  • .github/workflows/unittest.yml

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[gemini-code-assist]

Code Review

This PR updates the CI scripts to support running against different target branches, such as preview, by correctly parameterizing the target branch. However, it introduces a security risk due to several instances of unquoted shell variables in ci/run_conditional_tests.sh, particularly TARGET_BRANCH in git fetch commands, which could lead to command injection. Unquoted variables also make the script fragile. It is recommended to quote all shell variables for robustness and security, and use bash-specific constructs like [[ ... ]] for comparisons.