ci(build): Check the unit file made is valid.

While it isn't feasible to run the executable (due to it not having any
way of stopping itself) I can verify the unit file after making the
project.
So at least one thing is being tested.

.gitea/workflows/build.yaml

@@ -19,3 +19,6 @@ jobs:
 
       - name: Run make
         run: make
+
+      - name: Test unit file
+        run: systemd-analyze verify dexedrine.service