github bump-message: escape commit lock using tilde (#35)

This should fix the loss of backticks when echoing the script output to
the GHA result.

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/bump-gluon-commit-message.sh

@@ -27,7 +27,7 @@ build-info: update Gluon to $new_commit_date
 
 Update Gluon from $old_commit_short to $new_commit_short.
 
-\`\`\`
+~~~
 $commit_log
-\`\`\`
+~~~
 EOF