Improve gitbriefcase release check coverage

[rogerchappel]

What changed

• Hardened release:check so release dry runs exercise typecheck/check, tests, smoke coverage, and package packing through the canonical npm script.
• Made tgz creation portable on GNU tar by using GZIP=-n instead of the BSD-only tar --options gzip:!timestamp flag.

Why it matters

This makes PR and release evidence harder to accidentally weaken, and fixes the Linux CI failure where tar rejected --options during bundle creation tests.

Verification

• npm ci --no-audit --no-fund passed.
• npm run release:check passed.

Remaining limitations

• No package was published and no release tag was created.