build-node: Allow powerpc64le-linux builds.

* build-node.scm <cuirass-remote-worker-configuration>: Add
powerpc64le-linux to ‘systems’.

build-node.scm

@@ -83,7 +83,8 @@
                            ;; address, and (2) provide the port number.
                            (server "194.199.1.1:5555")
                            (systems (list "x86_64-linux"
-                                          "aarch64-linux"))))
+                                          "aarch64-linux"
+                                          "powerpc64le-linux"))))
 
                  (service avahi-service-type)     ;for Cuirass