services: guix-build-coordinator: Fix service activation parts.

Don't hardcode usernames, as these are in the config. Also fix the %user being
missing from the queue-builds service activation.

* gnu/services/guix.scm (guix-build-coordinator-activation,
guix-build-coordinator-agent-activation): Use config for the user name.
(guix-build-coordinator-queue-builds-activation): Define %user.

gnu/services/guix.scm

@@ -329,7 +329,9 @@
   #~(begin
       (use-modules (guix build utils))
 
-      (define %user (getpw "guix-build-coordinator"))
+      (define %user
+        (getpw #$(guix-build-coordinator-configuration-user
+                  config)))
 
       (chmod "/var/lib/guix-build-coordinator" #o755)
 
@@ -438,7 +440,9 @@
   #~(begin
       (use-modules (guix build utils))
 
-      (define %user (getpw "guix-build-coordinator-agent"))
+      (define %user
+        (getpw #$(guix-build-coordinator-agent-configuration-user
+                  config)))
 
       (mkdir-p "/var/log/guix-build-coordinator")
 
@@ -514,6 +518,10 @@
   #~(begin
       (use-modules (guix build utils))
 
+      (define %user
+        (getpw #$(guix-build-coordinator-queue-builds-configuration-user
+                  config)))
+
       (mkdir-p "/var/log/guix-build-coordinator")
 
       ;; Allow writing the PID file