workflows: improve test-ability in forks

Removing as many "owner == NixOS" conditions as possible allows easier
testing of CI itself in forks.

.github/workflows/backport.yml

@@ -14,7 +14,7 @@ permissions: {}
 jobs:
   backport:
     name: Backport Pull Request
-    if: github.repository_owner == 'NixOS' && github.event.pull_request.merged == true && (github.event.action != 'labeled' || startsWith(github.event.label.name, 'backport'))
+    if: vars.NIXPKGS_CI_APP_ID && github.event.pull_request.merged == true && (github.event.action != 'labeled' || startsWith(github.event.label.name, 'backport'))
     runs-on: ubuntu-24.04-arm
     steps:
       # Use a GitHub App to create the PR so that CI gets triggered

.github/workflows/check-cherry-picks.yml

@@ -16,7 +16,6 @@ jobs:
   check:
     name: cherry-pick-check
     runs-on: ubuntu-24.04-arm
-    if: github.repository_owner == 'NixOS'
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:

.github/workflows/manual-nixos-v2.yml

@@ -41,7 +41,6 @@ jobs:
           extra_nix_config: sandbox = true
 
       - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
-        if: github.repository_owner == 'NixOS'
         with:
           # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
           name: nixpkgs-ci

.github/workflows/manual-nixpkgs-v2.yml

@@ -28,7 +28,6 @@ jobs:
           extra_nix_config: sandbox = true
 
       - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
-        if: github.repository_owner == 'NixOS'
         with:
           # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
           name: nixpkgs-ci