ci: Add GitHub artifact attestations to package distribution

* Add generation of GitHub artifact attestations to built sdist and wheel
  before upload.
  c.f.:
   - https://github.blog/2024-05-02-introducing-artifact-attestations-now-in-public-beta/
   - https://docs.github.com/en/actions/security-guides/using-artifact-attestations-to-establish-provenance-for-builds

.github/workflows/wheels.yml

@@ -138,6 +138,8 @@ jobs:
       url: https://pypi.org/p/boost-histogram
     permissions:
       id-token: write
+      attestations: write
+      contents: read
 
     steps:
       - uses: actions/download-artifact@v4
@@ -149,4 +151,9 @@ jobs:
       - name: List all files
         run: ls -lh dist
 
+      - name: Generate artifact attestation for sdist and wheels
+        uses: actions/attest-build-provenance@173725a1209d09b31f9d30a3890cf2757ebbff0d # v1.1.2
+        with:
+          subject-path: "dist/boost_histogram-*"
+
       - uses: pypa/gh-action-pypi-publish@release/v1