Test that we can always extract strings (#1458)

This adds a check that `mdbook-xgettext` can always extract the strings
of the course. Without this, it’s possible to merge a change which will
make `mdbook-xgettext` error out the next time a translator tries to
refresh their translation.

.github/workflows/build.yml

@@ -122,7 +122,6 @@ jobs:
         uses: ./.github/workflows/setup-rust-cache
 
       - name: Install Gettext
-        if: matrix.language != 'en'
         run: |
           sudo apt update
           sudo apt install gettext
@@ -134,6 +133,12 @@ jobs:
         if: matrix.language != 'en'
         run: msgfmt --statistics -o /dev/null po/${{ matrix.language }}.po
 
+      - name: Test extracting English strings
+        if: matrix.language == 'en'
+        run: |
+          MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot"}}' mdbook build -d po
+          msgfmt -o /dev/null --statistics po/messages.pot
+
       - name: Build course
         run: mdbook build