Use a custom GitHub token in the backport action

Due to a limitation of the GitHub Actions default tokens, Pull Requests
opened with them won't trigger any events. Hence, they won't trigger the
CI builds. Use a custom token to circumvent that and let backport PRs be
built on CI.

Signed-off-by: Tom Wieczorek <twieczorek@mirantis.com>
(cherry picked from commit 97d26b2)
(cherry picked from commit e019731)

.github/workflows/backport.yaml

@@ -36,7 +36,7 @@ jobs:
         uses: zeebe-io/backport-action@v0.0.8
         with:
           # Config README: https://github.com/zeebe-io/backport-action#backport-action
-          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_token: ${{ secrets.GH_BACKPORT_TOKEN }}
           github_workspace: ${{ github.workspace }}
           # should be kept in sync with `uses`
           version: v0.0.8
@@ -48,4 +48,4 @@ jobs:
 
           pull_description: |-
             Automated backport to `${target_branch}`, triggered by a label in #${pull_number}.
-            See ${issue_refs}.
+            See ${issue_refs}.