chore(yarn-install): upgraded actions/cache action, deduped yarn version check

[fearphage]

• Upgraded actions/cache to the latest (v6.1.0)
  This version was released June 26.
• Reduced yarn version checks to a single invocation that sets the appropriate variables for future steps

Motivation

Node v20 is deprecated. This will get rid of some of the following warnings:

[freben]

Thanks for updating the cache action. The cache pin itself resolves correctly to actions/cache v6.1.0, but the Yarn refactor has blocking issues:

• The Yarn 1 install flag is misspelled as --fronzen-lockfile.
• The modern Yarn branch changes --immutable to the deprecated --frozen-lockfile alias; it should retain --immutable.
• The existing quoted regex, [[ "$(yarn --version)" =~ "1.*" ]], does not match a normal version such as 1.22.22. That sends Yarn 1 through the modern branch, where yarn config get cacheFolder may return undefined.

Please capture the version once, use an anchored Yarn 1 check such as ^1., keep --frozen-lockfile for Yarn 1 and --immutable for modern Yarn, and add or run smoke coverage for both paths. The fork CI workflow also still needs approval and a real run.

[fearphage]

@freben Updated. I tested this one locally:

➜  backstage-actions git:(upgrade-cache-action) bash -c 'for x in 1.2.3 2.3.4 10.11.12; do if [[ "$x" =~ ^1\. ]]; then echo "matched -> $x"; else echo "NO go -> $x"; fi; done'
matched -> 1.2.3
NO go -> 2.3.4
NO go -> 10.11.12