Add EPOCH_BUILD to latest setup step. (#7894)

Previously this was in setup-base. However, setup-base is no longer
used by the setup job.

Fixes https://github.com/meshtastic/gh-action-firmware/issues/10
Fixes https://github.com/meshtastic/firmware/issues/7888

.github/workflows/main_matrix.yml

@@ -48,6 +48,10 @@ jobs:
           python-version: 3.x
           cache: pip
       - run: pip install -U platformio
+      - name: Uncomment build epoch
+        shell: bash
+        run: |
+          sed -i 's/#-DBUILD_EPOCH=$UNIX_TIME/-DBUILD_EPOCH=$UNIX_TIME/' platformio.ini
       - name: Generate matrix
         id: jsonStep
         run: |