build: install: Do not set store GID.

There's no need to set the store GID as is will be done by the guix-daemon,
with the following snippet:

  if (chown(chrootStoreDir.c_str(), 0, buildUser.getGID()) == -1)
        throw SysError(format("cannot change ownership of ‘%1%’") % chrootStoreDir);

* gnu/build/install.scm (directives): Do not set store GID.

gnu/build/install.scm

@@ -101,9 +101,7 @@ (define* (evaluate-populate-directive directive target
 (define (directives store)
   "Return a list of directives to populate the root file system that will host
 STORE."
-  `(;; Note: the store's GID is fixed precisely so we can set it here rather
-    ;; than at activation time.
-    (directory ,store 0 30000 #o1775)
+  `((directory ,store 0 0 #o1775)
 
     (directory "/etc")
     (directory "/var/log")                          ; for shepherd