Change master to trunk in github workflow

The driver donation resulted in a change of the main branch name but this change wasn't made in the github workflow. This patch fixes that.

Patch by João Reis; reviewed by James Hartig for CASSGO-85

Author: joao-r-reis

.github/workflows/main.yml

@@ -3,7 +3,7 @@ name: Build
 on:
   push:
     branches:
-      - master
+      - trunk
   pull_request:
     types: [ opened, synchronize, reopened ]