fix(macos): build x86_64 with the standalone spc binary - #2

• The pinned SPC_MACOS_X86_64_SHA256 equals the GitHub-published digest of spc-macos-x86_64.tar.gz in the static-php-cli 2.8.5 release.
• The tarball is now written to disk and checked with shasum -a 256 -c before tar extracts it, so a mismatch fails the step before anything runs.
• Extracting straight into spc/bin gives spc/bin/spc, the same path the old mv spc/spc spc/bin/spc produced, matching the later ./bin/spc calls.

No automated test covers the workflow change. The only evidence is the manual dispatch run linked in the PR description, plus the spc/bin/spc --version smoke check inside the download step.

• Commits: 253e3e7..a67f105
• Model: claude-opus-5-5