[gha/test] Add canary branch for external contributors build lint (#992)

Canary for #991 

It works because in this stack, this PR is targeting the branch `06-18-_gha_allow_external_contributors_build_lint`, so under `pull_request_target`, it will use the `ci.yml` workflow file from that branch.

## Test plan

CI works, and Semgrep passes

Author: rustielin

.github/workflows/ci.yml

@@ -7,6 +7,7 @@ on:
   pull_request_target:
     branches:
       - 06-18-_gha_allow_external_contributors_build_lint # canary for the PR that introduces pull_request_target
+      - 06-18-_gha_canary_allow_external_contributors_build_lint
 
 permissions:
   contents: read