chore: ref v1.8.0 instead of main (slsa-framework#2509)

Similar to
slsa-framework#2482 (comment)

cc @laurentsimon

Signed-off-by: AdamKorcz <[email protected]>
Signed-off-by: Noah Elzner <[email protected]>

Author: AdamKorcz

internal/builders/maven/action.yml

@@ -62,7 +62,7 @@ runs:
       uses: slsa-framework/slsa-github-generator/.github/actions/secure-builder-checkout@main
       with:
         repository: slsa-framework/slsa-github-generator
-        ref: main
+        ref: v1.8.0
         path: __BUILDER_CHECKOUT_DIR__
     - name: Run mvn package
       shell: bash