feat(ci): don't run checks outside main repo

.github/workflows/checks.yml

@@ -12,6 +12,7 @@ jobs:
     steps:
       - name: Check if PRs have conflicts
         uses: eps1lon/actions-label-merge-conflict@v3
+        if: github.repository == ${{ vars.REPO_MAIN }}
         with:
           dirtyLabel: "merge-conflicts"
           repoToken: "${{ secrets.MERGE_CONFLICT_LABEL_PAT }}"

.github/workflows/nightly.yml

@@ -27,7 +27,7 @@ permissions:
 
 jobs:
   nightly-electron:
-    if: github.repository == 'TriliumNext/Trilium'
+    if: github.repository == ${{ vars.REPO_MAIN }}
     name: Deploy nightly
     strategy:
       fail-fast: false
@@ -98,7 +98,7 @@ jobs:
           path: apps/desktop/upload
 
   nightly-server:
-    if: github.repository == 'TriliumNext/Trilium'
+    if: github.repository == ${{ vars.REPO_MAIN }}
     name: Deploy server nightly
     strategy:
       fail-fast: false