dont use nix build on every PR

.github/workflows/nix-build.yml

@@ -11,10 +11,6 @@ on:
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
 
-  # TODO: remove before merging to main — only here so pushes to this
-  # branch exercise the workflow while it's being tested.
-  pull_request:
-
 jobs:
   build:
     name: nix build (${{ matrix.system }})