[ci] Build ragel on the Linux CI job

To make sure we don’t accidentally break this option.
diff --git a/.github/workflows/linux-ci.yml b/.github/workflows/linux-ci.yml
index 5a82f7d..0d841ca 100644
--- a/.github/workflows/linux-ci.yml
+++ b/.github/workflows/linux-ci.yml
@@ -13,10 +13,10 @@
     steps:
     - uses: actions/checkout@v2
     - name: install dependencies
-      run: sudo apt-get install pkg-config gcc ragel gcovr gtk-doc-tools libfreetype6-dev libglib2.0-dev libcairo2-dev libicu-dev libgraphite2-dev python3 python3-setuptools ninja-build gobject-introspection libgirepository1.0-dev
+      run: sudo apt-get install pkg-config gcc gcovr gtk-doc-tools libfreetype6-dev libglib2.0-dev libcairo2-dev libicu-dev libgraphite2-dev python3 python3-setuptools ninja-build gobject-introspection libgirepository1.0-dev
     - run: sudo pip3 install fonttools meson==0.52.0
     - name: run
-      run: meson build -Db_coverage=true --auto-features=enabled -Dgraphite=enabled -Dchafa=disabled -Doptimization=2
+      run: meson build -Db_coverage=true --auto-features=enabled -Dgraphite=enabled -Dchafa=disabled -Dragel_subproject=true -Doptimization=2
     - name: ci
       run: meson test --print-errorlogs -Cbuild