ci: Linux IS #1850 release build back into rust-regression-linux; warm macOS release deps on main #3186

Problem

After #3184 split the IS #1850 release build out of rust-regression-linux, the new issue-1850-release-linux job spent ~45 min in Install Tauri system dependencies on both PR #3185 runs: apt fetched ~86 MB at ~32 kB/s from azure.archive.ubuntu.com, while rust-regression-linux, clippy-linux and rust-release-parity-linux fetched the same packages at 11-12 MB/s in the same runs, on the same image. The step is identical; the cause is not known.

The release build itself takes 7-15 min on Linux, so running it in parallel saves little: the full tier's longest job is issue-1850-release-macos (~52 min cold).

Change

Move the IS #1850 release build steps back into rust-regression-linux, where they ran before #3184, and remove issue-1850-release-linux.

Ruleset follow-up (at merge)

Remove issue-1850-release-linux from the required checks of the main ruleset.

Also: warm the macOS release dependencies on main

PR regression gates never runs on main, so issue-1850-release-macos compiles its release dependencies cold on every pull request (52.6 min on the second PR #3185 run). Add warm-release-macos and verify-warm-release-macos to cache-warm.yml with shared key gate-release-macos, and make issue-1850-release-macos restore it (save-if: false), like gate-release on Windows. Only dependencies are cached; the crate itself (~33 min in release on macOS) still compiles on every run.