Install Ghostscript in Github Actions workflow

.github/workflows/main.yml

@@ -14,6 +14,11 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
+      - name: Install additional packages
+        run: |
+          set -ex
+          apt-get -qy update
+          apt-get -qy install ghostscript
       - name: Install the Fithesis package
         run: |
           set -ex