ci: fix the self-hosted Release Build's Xcode, XcodeGen and fallback

[datlechin]

Why

Follow-ups to #3550 for Release Build on the self-hosted Mac:

• The Mac has Xcode 27.1 beta 27A9269; the image and the tag use the released 27A9275. A version-only match let another compiler pass the gate.
• setup-xcodegen used the Mac's unverified Homebrew xcodegen, and its fallback needs sudo, which the runner lacks.
• Only a dispatch could leave the Mac, so a release branch push blocked the tag while it was offline.

Change

• setup-xcode pins the build as well as the version, on every job.
• setup-xcodegen installs the checked archive into the runner's tool cache, no sudo, via GITHUB_PATH.
• runs-on also reads the RELEASE_BUILD_RUNNER repository variable; RELEASING.md, When a run fails, says when to set it.

Verified

Both action scripts ran on the Mac: setup-xcode rejects 27A9269; XcodeGen installs without sudo and loads its presets. actionlint and shellcheck clean.

Not in this PR

• The Mac needs 27A9275 at /Applications/Xcode.app; until then its nightly fails in setup-xcode.