ci: don't run linux on push to non-master branches

It's redundant with the pull request run.

Signed-off-by: Ran Benita <ran@unusedvar.com>

.github/workflows/linux.yml

@@ -1,6 +1,10 @@
 name: linux
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
 
 # Set permissions at the job level.
 permissions: {}