(DO NOT MERGE) BUILD-10706: Configure gh-action_cache to use S3 cache backend

[bwalsh434]

Use S3 cache backend for CI caching.

Relates to https://sonarsource.atlassian.net/browse/BUILD-10706 (parent ticket - BUILD-10684)

[hashicorp-vault-sonar-prod]

BUILD-10706

[sonar-review-alpha]

Summary

Configures GitHub Actions workflows to use S3 as the cache backend instead of the default storage. The change adds a CACHE_BACKEND: s3 environment variable to two workflows: the main build pipeline and the unified-dogfooding scheduled job. This enables CI jobs to leverage S3 for caching build artifacts, dependencies, and other intermediate outputs.

What reviewers should know

The changes are minimal—just environment variable declarations in two workflow files. To review effectively: (1) verify that the build system actually respects the CACHE_BACKEND variable and is configured to handle S3 in the build scripts/Gradle configuration, (2) check that S3 access credentials are already set up as GitHub secrets or runner environment, since this PR doesn't add those, and (3) ensure the S3 bucket and IAM permissions are in place. The changes have no direct runtime impact on job logic, only cache behavior.

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[sonarqube-next]

Quality Gate passed for 'Ruby'

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