Exit early in metrics tests script

[mokagio]

Related issues

#2310

Proposed Changes

I propose to front load the check for PR or main branch in the CI step running the metrics tests, so that if the CI build is on neither PR nor main branch, we can immediately bypass the step.

Testing Instructions

I run https://buildkite.com/automattic/studio/builds/10136 manually via the Buildkite UI before opening this PR. Notice the tests were skipped.

Compare with the https://buildkite.com/automattic/studio/builds/10137, automatically triggered when the the PR was opened, where the metrics test and their ~35s setup run.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 7c0a295 vs trunk

site-editor

Metric	trunk	7c0a295	Diff	Change
load	6373.50 ms	7242.00 ms	+868.50 ms	🔴 13.6%

site-startup

Metric	trunk	7c0a295	Diff	Change
siteCreation	9071.50 ms	9084.50 ms	+13.00 ms	🔴 0.1%
siteStartup	3951.50 ms	3947.00 ms	-4.50 ms	🟢 -0.1%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[wojtekn]

Closing as I implemented this condition on the pipeline level. Thanks for the idea!