Add missing permission to check_libs.yaml (#7)

Repository is configured to use permission-less GITHUB_TOKEN by default
(instead of granting write for all permissions)

i.e. "Read repository contents and packages permissions" instead of
"Read and write permissions" for "Workflow permissions" setting
> Choose the default permissions granted to the GITHUB_TOKEN when
running workflows in this repository. You can specify more granular
permissions in the workflow using YAML.

Author: carlcsaposs-canonical

.github/workflows/check_libs.yaml

@@ -37,3 +37,5 @@ jobs:
           charm-path: ${{ matrix.path }}
           credentials: "${{ secrets.CHARMHUB_TOKEN }}"
           github-token: "${{ secrets.GITHUB_TOKEN }}"
+    permissions:
+      issues: write  # Needed to edit pull request labels