test(perfmetrics): support workflow-specific parallel execution in presubmit

[PranjalC100]

Description

Supports granular workflow routing in perfmetrics/scripts/presubmit_test/pr_perf_test/build.sh via the PRESUBMIT_WORKFLOW environment variable ("regional", "zonal", "perf", "other", or default "all").

• Enables Kokoro to run presubmit test suites concurrently across dedicated VMs instead of executing them sequentially on a single VM.
• Includes fast-exit checks (<3s) that query PR labels via GitHub API and immediately exit 0 when a requested workflow's label is absent.
• Preserves 100% backward compatibility when PRESUBMIT_WORKFLOW is unset.

Link to the issue in case of a bug fix.

Fixes b/446921841

Testing details

1. Manual - Simulated all 11 workflow combinations (fast-exits and positive label matches for perf, regional, zonal, other, and default all) with mock GitHub label payloads; verified all exited with code 0.
2. Unit tests - Verified go build ./... and go fmt ./....
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

No. When PRESUBMIT_WORKFLOW is not specified, it defaults to "all" and behaves identically to existing master runs.

[gemini-code-assist]

Code Review

This pull request refactors the presubmit script (build.sh) to support running specific test workflows (perf, regional, zonal, other, or all) based on a new PRESUBMIT_WORKFLOW environment variable, and introduces fast-exit conditions for each workflow type. Feedback was provided regarding critical issues with the curl command fetching PR details, which could silently bypass tests on failure, append to stale data, or fetch incorrect PRs if the PR number is unset.