workflows: build redfishpower in distcheck

Problem: The t0029-redfish.t test now uses the redfishpower
--test-mode option, not the redfishpower simulator.  This
requires 'make distcheck' to build redfishpower.

Set the DISTCHECK_CONFIGURE_FLAGS environment variable to
ensure distcheck builds redfishpower.

.github/workflows/main.yml

@@ -47,7 +47,7 @@ jobs:
     - name: make check
       run: make check
     - name: make distcheck
-      run: make distcheck
+      run: DISTCHECK_CONFIGURE_FLAGS=--with-redfishpower make distcheck
 
   spelling:
     runs-on: ubuntu-20.04