ci: Set BASE_URL required for MyST html build (#113)

* The 'x build --html' of myst and jupyter-book require the environmental
  variable 'BASE_URL' to be set.
   - c.f. https://mystmd.org/guide/deployment#deploy-base-url
   - c.f. https://mystmd.org/guide/deployment-github-pages#base-url-configuration-for-github-pages

Author: matthewfeickert

.github/workflows/deploy-jupyter-book.yml

@@ -22,6 +22,10 @@ jobs:
   build-book:
     runs-on: ubuntu-latest
 
+    env:
+      # Set BASE_URL required for build --html
+      BASE_URL: /${{ github.event.repository.name }}
+
     steps:
     - uses: actions/checkout@v6