.github/workflows/manual-{nixos,nixpkgs}.yml: add `repository_owner` condition

We have this set in the other actions, it prevents the action from
running in PRs made against forks.

.github/workflows/manual-nixos.yml

@@ -12,6 +12,7 @@ on:
 jobs:
   nixos:
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'NixOS'
     steps:
       - uses: actions/checkout@v2
         with:

.github/workflows/manual-nixpkgs.yml

@@ -12,6 +12,7 @@ on:
 jobs:
   nixpkgs:
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'NixOS'
     steps:
       - uses: actions/checkout@v2
         with: