github: trigger all builds on main branch, and PR for any branch

The conditions for triggering Nix builds and other builds were
slightly different.

Nix builds triggered by PRs happened on PRs for any branch, not just
the `main` branch. That makes very little difference in practice
because PRs for other branches are very rare. Still, let's be
consistent. I decided to trigger the builds on PRs for any branch.

More importantly, Nix builds triggered by push were only done for
pushes to `master`, which is not what our main branch is called, so
those never happened.

.github/workflows/build.yml

@@ -2,9 +2,9 @@ name: build
 
 on:
   push:
-    branches: [ main ]
+    branches:
+      - main
   pull_request:
-    branches: [ main ]
 
 permissions: read-all
 

.github/workflows/nix-linux.yml

@@ -3,7 +3,7 @@ name: Nix on Linux
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
 
 permissions: read-all