CI: Remove workaround of setting the environment variables

[pentamassiv]

Because gtk4-layer-shell is built and installed from source, the lib is in a different path. Two environment variables need to be set to make the builds work. Remove this from the CI once gtk4-layer-shell is installed as a package

This is the code to be removed:

      - name: Export environment variables
        run: |
          export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
          export LD_LIBRARY_PATH=/usr/local/lib