Pithos CI: install the gateway's dependencies for its tests

test/theseus-id.test.js imports hosted/server.js, which needs
@bitauth/libauth from hosted/node_modules; the runner only installed the
app's own dependencies, so the job failed before building.

.forgejo/workflows/desktop-linux.yml

@@ -16,7 +16,10 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Install
-        run: npm ci --no-audit --no-fund
+        # hosted/ too: test/theseus-id.test.js runs the real gateway
+        run: |
+          npm ci --no-audit --no-fund
+          npm ci --no-audit --no-fund --prefix hosted
 
       - name: Tests
         run: npm test