build(deps): bump sigstore/sigstore-conformance in the actions group (#1357)

Bumps the actions group with 1 update: [sigstore/sigstore-conformance](https://github.com/sigstore/sigstore-conformance).


Updates `sigstore/sigstore-conformance` from 0.0.17 to 0.0.18
- [Release notes](https://github.com/sigstore/sigstore-conformance/releases)
- [Commits](sigstore/sigstore-conformance@640e7df...fd90e6b)

---
updated-dependencies:
- dependency-name: sigstore/sigstore-conformance
  dependency-version: 0.0.18
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: actions
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/conformance.yml

@@ -24,7 +24,7 @@ jobs:
       - name: install sigstore-python
         run: python -m pip install .
 
-      - uses: sigstore/sigstore-conformance@640e7dfb715518eeeb492910c6d244cedcc6cfea # v0.0.17
+      - uses: sigstore/sigstore-conformance@fd90e6b0f3046f2276a6659481de6df495dea3b9 # v0.0.18
         with:
           entrypoint: ${{ github.workspace }}/test/integration/sigstore-python-conformance
           xfail: "test_verify_with_trust_root test_verify_dsse_bundle_with_trust_root" # see issue 821