github actions: change default branch from master to main

Author: iwanbk

.github/workflows/build.yaml

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