CI (Windows): makes signing policy configurable

Author: CarterLi

.github/workflows/ci.yml

@@ -725,7 +725,7 @@ jobs:
           api-token: '${{ secrets.SIGNPATH_API_TOKEN }}'
           organization-id: '${{ vars.SIGNPATH_ORG_ID }}'
           project-slug: 'fastfetch'
-          signing-policy-slug: 'test-signing'
+          signing-policy-slug: '${{ vars.SIGNPATH_SIGNING_POLICY }}'
           github-artifact-id: '${{ steps.upload-unsigned-artifact.outputs.artifact-id }}'
           wait-for-completion: true
           output-artifact-directory: '.'