CI: do not run Bump dependencies workflow on forks

.github/workflows/dependencies.yml

@@ -25,6 +25,7 @@ env:
 
 jobs:
   not-waiting-on-bors:
+    if: github.repository_owner == 'rust-lang'
     name: skip if S-waiting-on-bors
     runs-on: ubuntu-latest
     steps:
@@ -43,6 +44,7 @@ jobs:
           fi
 
   update:
+    if: github.repository_owner == 'rust-lang'
     name: update dependencies
     needs: not-waiting-on-bors
     runs-on: ubuntu-latest
@@ -76,6 +78,7 @@ jobs:
           retention-days: 1
 
   pr:
+    if: github.repository_owner == 'rust-lang'
     name: amend PR
     needs: update
     runs-on: ubuntu-latest