.github/workflows/basic-eval.yml: only run if manually triggered

ofborg should have obsoleted this with https://github.com/NixOS/ofborg/pull/588#issuecomment-1078451552
https://github.com/NixOS/nixpkgs/pull/166599#issuecomment-1085069938

this is still useful to have around for act usage

.github/workflows/basic-eval.yml

@@ -1,14 +1,15 @@
 name: Basic evaluation checks
 
 on:
-  pull_request:
-    branches:
-     - master
-     - release-**
-  push:
-    branches:
-     - master
-     - release-**
+  workflow_dispatch
+  # pull_request:
+  #   branches:
+  #    - master
+  #    - release-**
+  # push:
+  #   branches:
+  #    - master
+  #    - release-**
 jobs:
   tests:
     runs-on: ubuntu-latest