fix(ci): restore deleted workflow_call trigger

This is required to call this workflow from another workflow, such as
the release.yml workflow.

.github/workflows/main.yml

@@ -5,6 +5,7 @@ on:
     branches: [ main ]
   pull_request:
     branches: [ main ]
+  workflow_call:
 
 env:
   CARGO_TERM_COLOR: always