Grant export workflows read/write permission

Signed-off-by: Jon Johnson <jon.johnson@chainguard.dev>

Author: jonjohnsonjr

.github/chainguard/stereo.sts.yaml

@@ -0,0 +1,11 @@
+issuer: https://token.actions.githubusercontent.com
+
+subject_pattern: repo:chainguard-dev/stereo:ref:refs/heads/main
+
+claim_pattern:
+  job_workflow_ref: chainguard-dev/stereo/.github/workflows/(export|publish)-wolfi.yaml@refs/heads/main
+
+permissions:
+  contents: write
+  pull_requests: write
+  workflows: write