Use mdbook-i18n-helpers crate (#552)

The i18n-helpers are now available as a stand-alone crate:
https://crates.io/crates/mdbook-i18n-helpers.

Because we cache the Rust binaries in our GitHub workflows, I bumped
the cache prefix to ensure we use a clean cache. Otherwise, Cargo
won’t install the new binaries in mdbook-i18n-helpers because it sees
the old ones from this repository.

.github/workflows/build.yml

@@ -86,43 +86,6 @@ jobs:
         working-directory: ${{ matrix.directory }}
         run: cargo build
 
-  i18n-helpers:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v3
-
-      - name: Setup Rust cache
-        uses: ./.github/workflows/setup-rust-cache
-
-      - name: Install Gettext
-        run: sudo apt install gettext
-
-      - name: Install mdbook
-        uses: ./.github/workflows/install-mdbook
-
-      - name: Generate po/messages.pot
-        run: mdbook build -d po
-        env:
-          MDBOOK_OUTPUT: '{"xgettext": {"pot-file": "messages.pot"}}'
-
-      - name: Test messages.pot
-        run: msgfmt --statistics -o /dev/null po/messages.pot
-
-      - name: Expand includes without translation
-        run: mdbook build -d expanded
-        env:
-          MDBOOK_OUTPUT: '{"markdown": {}}'
-
-      - name: Expand includes with no-op translation
-        run: mdbook build -d no-op
-        env:
-          MDBOOK_OUTPUT: '{"markdown": {}}'
-          MDBOOK_PREPROCESSOR__GETTEXT__PO_FILE: po/messages.pot
-
-      - name: Compare no translation to no-op translation
-        run: diff --color=always --unified --recursive expanded no-op
-
   find-translations:
     runs-on: ubuntu-latest
     outputs:

.github/workflows/install-mdbook/action.yml

@@ -1,6 +1,6 @@
 name: Install mdbook and dependencies
 
-description: Install the mdbook with the dependencies we need.
+description: Install mdbook with the dependencies we need.
 
 runs:
   using: composite
@@ -15,6 +15,6 @@ runs:
       run: cargo install mdbook-svgbob --locked --version 0.2.1
       shell: bash
 
-    - name: Install i18n-helpers
-      run: cargo install --path i18n-helpers --locked
+    - name: Install mdbook-i18n-helpers
+      run: cargo install mdbook-i18n-helpers --locked --version 0.1.0
       shell: bash

.github/workflows/setup-rust-cache/action.yml

@@ -7,3 +7,5 @@ runs:
   steps:
     - name: Setup Rust cache
       uses: Swatinem/rust-cache@v2
+      with:
+         prefix-key: v1  # Remove after merging #1322.