ci: attest release archives

goreleaser has no attestation of its own — the documented route is
actions/attest after it runs. subject-checksums attests every file listed in
checksums.txt, so all six archives are covered without naming them.

Verify with `gh attestation verify --owner Flagsmith <file>`.

beep boop

Author: khvn26

.github/workflows/release.yml

@@ -12,6 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write # upload release artifacts
+      id-token: write # attest artifacts
+      attestations: write
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
@@ -26,3 +28,7 @@ jobs:
           args: release --clean
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      # Attests every file listed in the checksum file.
+      - uses: actions/attest@508db95dd578ae2727ebd6217d5ba78e4fbda05d # v4.2.1
+        with:
+          subject-checksums: ./dist/checksums.txt