ci: disable pr trigger event for sigstore workflow

The sigstore workflow requires id-token: 'write' permission to sign
with ambient credentials. This permission is not available in a
PRs from forks.

Signed-off-by: Lukas Puehringer <[email protected]>

Author: lukpueh

.github/workflows/test-sigstore.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - main
-  pull_request:
   workflow_dispatch:
 
 permissions: {}