ci(actions): fix mix not found

Author: mydearxym

.github/workflows/ci.yml

@@ -73,7 +73,11 @@ jobs:
       - name: Check Bundle Size
         run: BUNDLEWATCH_GITHUB_TOKEN=${{ secrets.BUNDLEWATCH_GITHUB_TOKEN }} npm run size.check
       - name: Run Backend Server
+        uses: actions/setup-elixir@v1
+        with:
+          elixir-version: '1.10.3' # Define the elixir version [required]
+          otp-version: '22.3' # Define the OTP version [required]
         working-directory: ./server
-        run: MIX_ENV=ci mix phx.server
+        run: MIX_ENV=ci mix phx.server &
       - name: Run Tests
         run: npm run test:ci