[ci] Fix to actually run on pushing to main branch

A reasonable assumption would be that `$default-branch` would refer to
the default branch of the repository, when the workflow runs. However,
this is not the case: the `$default-branch` variable is intended for
workflow **templates** only, and gets macro-expanded when copying the
workflow; **not** when running it. C.f.
actions/starter-workflows#590 (comment)

Author: CodingCellist

.github/workflows/pelican.yml

@@ -9,7 +9,8 @@ name: Deploy Pelican site to GH Pages
 
 on:
   push:
-    branches: [$default-branch]
+    branches:
+      - main
 
   workflow_dispatch: