SONARJAVA-6253 Ensure integration tests do not share cache

[tomasz-tylenda-sonarsource]

Use separate caches for plugin, ruling, and other ITs. See SONARJAVA-6253 for details.

[hashicorp-vault-sonar-prod]

SONARJAVA-6253

[sonar-review-alpha]

Summary

This PR ensures that separate integration test jobs don't share Maven caches by introducing a unique cache key per job. A new GitHub Action writes a file (just_for_cache_key_calculation/pom.xml) with job-specific content (job name and matrix variables) before Maven runs. Since GitHub's cache key includes the content hash of files in the cache path, each job now gets a distinct cache, preventing cross-job cache pollution.

What reviewers should know

How it works:

• New reusable action at .github/actions/write-file/ — a simple utility that writes content to a file
• Added to 6 integration test jobs in the build workflow (ruling, plugin, qa, qa-plugin, license-check, plugin-check)
• The file is written with unique per-job content: either ${{ github.job }} alone or ${{ github.job }}-${{ matrix variables }}
• This file becomes part of Maven's cache scope, so different jobs naturally get different caches

For reviewers:

• The approach is non-obvious but clever — it exploits how GitHub's cache hashing works rather than using explicit cache keys
• The file path just_for_cache_key_calculation/ makes the intent clear: it exists purely for cache separation, not as a real project file
• No changes to job logic or Maven configuration — this is purely a cache isolation mechanism
• All instances follow the same pattern; review one or two to understand them all

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[sonarqube-next]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
0 Dependency risks
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[sonar-review-alpha]

LGTM! ✅

Clean, focused change with no bugs. The approach of writing a unique-content file to influence Maven's cache hash is correct and each job is properly distinguished — non-matrix jobs by github.job, matrix jobs by github.job + matrix variables. Using env vars in the composite action (FILE_PATH, CONTENT) correctly avoids shell injection risks.

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[tomasz-tylenda-sonarsource]

I have an alternative fix in #5567 and SonarSource/ci-github-actions#244