MINOR: Add check-shebang-scripts-are-executable (#670)

## What's Changed

Make sure that new scripts committed are actually executable, because I
always forget.

Author: lidavidm

.github/workflows/dev_pr_milestone.sh

@@ -25,6 +25,7 @@ repos:
     hooks:
       - id: trailing-whitespace
       - id: end-of-file-fixer
+      - id: check-shebang-scripts-are-executable
       - id: check-yaml
       - id: check-added-large-files
       - id: file-contents-sorter