Format all PO files with dprint (#1359)

This enforces a consistent formatting for the PO files. The goal of this
is to avoid large diffs due to random and unnecessary reformatting.

We use the format of `msgcat`: this is also waht `msgmerge` produces and
it’s easy to replicate for people by installing Gettext and running
`dprint fmt`.

This is a follow-up to #1351 which started enforcing that the `msgid`
fields don’t change due to reformatting in a PR.

If this turns out to be cumbersome, then we can disable it again.

.github/workflows/build.yml

@@ -16,6 +16,10 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
 
+      - name: Install Gettext
+        run: |
+          sudo apt install gettext
+
       - name: Install nightly rustfmt
         run: |
           rustup default nightly