Installs libgd dependency for Linux builds

Adds a step to the Linux CI workflow to install the `libgd-dev` package.
This resolves a missing dependency issue preventing the build from completing
successfully on Linux.

Author: brillcp

.github/workflows/linux.yaml

@@ -8,7 +8,12 @@ jobs:
     container: swift:6.2.0
     steps:
       - uses: actions/checkout@v4
+
+      - name: Install libgd
+        run: apt-get update && apt-get install -y libgd-dev
+
       - name: Build
         run: swift build -v
+
       - name: Test
         run: swift test -v