ci: fix run conditions to the right branch

the Nix build CI job doesn't run at all, since it's pointing to a branch
that does not exist. Update it to run only on push to master branch (not
PRs, seems overkill since we can check the build locally).
Updates the new test.yml job to run on the master branch too.

.github/workflows/ci.yml

@@ -2,9 +2,7 @@ name: Build outputs
 
 on:
   push:
-    branches: [main]
-  pull_request:
-    branches: [main]
+    branches: [master]
 
 jobs:
   build-nix-outputs:

.github/workflows/test.yml

@@ -2,9 +2,9 @@ name: Run tests
 
 on:
   push:
-    branches: [main]
+    branches: [master]
   pull_request:
-    branches: [main]
+    branches: [master]
 
 jobs:
   run-tests: