Bump github.com/sigstore/sigstore from 1.8.3 to 1.8.4

Bumps [github.com/sigstore/sigstore](https://github.com/sigstore/sigstore) from 1.8.3 to 1.8.4.
- [Release notes](https://github.com/sigstore/sigstore/releases)
- [Commits](sigstore/sigstore@v1.8.3...v1.8.4)

---
updated-dependencies:
- dependency-name: github.com/sigstore/sigstore
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -18,7 +18,7 @@ require (
 	github.com/mitchellh/go-homedir v1.1.0
 	github.com/opencontainers/image-spec v1.1.0
 	github.com/pkg/errors v0.9.1
-	github.com/sigstore/sigstore v1.8.3
+	github.com/sigstore/sigstore v1.8.4
 	github.com/spiffe/go-spiffe/v2 v2.2.0
 	github.com/spiffe/spire-api-sdk v1.9.6
 	github.com/tektoncd/plumbing v0.0.0-20220817140952-3da8ce01aeeb