ci(build): allow the unit file to be tested.

As the container that runs the workflow logs in as root it is impossible
to test starting a user service without some work that I cba to do.
But I can at least verify that the unit file is valid

.gitea/workflows/build.yaml

@@ -19,3 +19,9 @@ jobs:
 
       - name: Run make
         run: make
+
+      - name: Install dexedrine to the container
+        run: sudo make install
+
+      - name: Test unit file
+        run: systemd-analyze verify dexedrine.service