ci: only run lock and regress workflows on the main repo

Forks are just wasting CI cycles spinning actions which are probably not
looked at. Also avoid locking issues and PRs on any forks with
upstream's policies.

.github/workflows/lock.yml

@@ -16,6 +16,7 @@ concurrency:
 jobs:
   action:
     runs-on: ubuntu-latest
+    if: github.repository == 'tmux/tmux'
     steps:
       - uses: dessant/lock-threads@v6
         with:

.github/workflows/regress.yml

@@ -16,6 +16,7 @@ jobs:
   regress:
     name: ${{ matrix.name }}
     runs-on: ${{ matrix.runner }}
+    if: github.repository == 'tmux/tmux'
     timeout-minutes: 45
 
     strategy: