Fix CI by running on `main` branch instead of `master`

Ugh I hadn't noticed that CI was broken, which didn't pick up that a PR
didn't actually build.

.github/workflows/build.yml

@@ -2,9 +2,9 @@ name: Build
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 env:
   CARGO_TERM_COLOR: always