ci: update s-z workflows to use labeled on pull_request trigger

[grayside]

Description

Scoped to Samples workflows with file names starting with s-z.

Implementation of #3747, sibling of #3748.

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[iennae]

ooh, good catch to make this more clear.

[grayside]

This was triggering GHA lint errors in vscode.

[grayside]

• scheduler: initial failure with timeout. manual retry fixed
• talent: test failure from sample bug, possibly a race condition? manual retry fixed
• speech: test failure. retry failed
• translate: failed. retry failed
• videointelligence: failed. retry failed
• vision: failed, retry failed
• vision-productSearch: failed, retry failed

[grayside]

These test failures are out of scope. Assembled notes to file follow-up bugs.

[iennae]

Thank you! acknowledging that the test failures will be resolved later.