build(deps): bump github.com/sigstore/sigstore from 1.8.12 to 1.8.15

Bumps [github.com/sigstore/sigstore](https://github.com/sigstore/sigstore) from 1.8.12 to 1.8.15.
- [Release notes](https://github.com/sigstore/sigstore/releases)
- [Commits](sigstore/sigstore@v1.8.12...v1.8.15)

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

Signed-off-by: dependabot[bot] <[email protected]>

go.mod

@@ -57,7 +57,7 @@ require (
 	github.com/redis/go-redis/v9 v9.7.0
 	github.com/rs/zerolog v1.33.0
 	github.com/sigstore/cosign/v2 v2.4.2
-	github.com/sigstore/sigstore v1.8.12
+	github.com/sigstore/sigstore v1.8.15
 	github.com/smartystreets/goconvey v1.8.1
 	github.com/spf13/cast v1.7.1
 	github.com/spf13/cobra v1.9.1

go.sum

@@ -1938,8 +1938,8 @@ github.com/sigstore/protobuf-specs v0.4.0 h1:yoZbdh0kZYKOSiVbYyA8J3f2wLh5aUk2SQB
 github.com/sigstore/protobuf-specs v0.4.0/go.mod h1:FKW5NYhnnFQ/Vb9RKtQk91iYd0MKJ9AxyqInEwU6+OI=
 github.com/sigstore/rekor v1.3.9 h1:sUjRpKVh/hhgqGMs0t+TubgYsksArZ6poLEC3MsGAzU=
 github.com/sigstore/rekor v1.3.9/go.mod h1:xThNUhm6eNEmkJ/SiU/FVU7pLY2f380fSDZFsdDWlcM=
-github.com/sigstore/sigstore v1.8.12 h1:S8xMVZbE2z9ZBuQUEG737pxdLjnbOIcFi5v9UFfkJFc=
-github.com/sigstore/sigstore v1.8.12/go.mod h1:+PYQAa8rfw0QdPpBcT+Gl3egKD9c+TUgAlF12H3Nmjo=
+github.com/sigstore/sigstore v1.8.15 h1:9HHnZmxjPQSTPXTCZc25HDxxSTWwsGMh/ZhWZZ39maU=
+github.com/sigstore/sigstore v1.8.15/go.mod h1:+Wa5mrG6A+Gss516YC9owy10q3IazqIRe0y1EoQRHHM=
 github.com/sigstore/sigstore-go v0.7.0 h1:bIGPc2IbnbxnzlqQcKlh1o96bxVJ4yRElpP1gHrOH48=
 github.com/sigstore/sigstore-go v0.7.0/go.mod h1:4RrCK+i+jhx7lyOG2Vgef0/kFLbKlDI1hrioUYvkxxA=
 github.com/sigstore/sigstore/pkg/signature/kms/aws v1.8.12 h1:EC3UmIaa7nV9sCgSpVevmvgvTYTkMqyrRbj5ojPp7tE=