CI: Give actions write permission to page contents

According to https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/
using pull_request_target will use the base branch instead of the fork
branch. The base branch is considered trusted, and the automatic token
handed over to GHA will include write permission.

Fixes #154

.github/workflows/pr.yml

@@ -1,7 +1,7 @@
 name: build-pr
 
 on:
-  pull_request:
+  pull_request_target:
     branches:
       - master