ci: fork-friendly GitHub Actions workflow fixes

Summary

Split from #876 (per review feedback): CI workflow changes only.

• rustfmt_check: use repository default branch instead of hard-coded main
• build: rust-cache@v2, fail-fast: false, upstream tag fetch, GITHUB_TOKEN for API auth

Test plan

• Push to fork and confirm rustfmt_check and build workflows run

Related: pairs with xtask/install-toolkit-github-auth for install-toolkit 403 fix.

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