services: virtual-build-machine: Add base file systems to default OS.

This provides /dev/pts and other things that are important for
builds (for example, Python 2.x has ‘openpty’ unit tests that can only
succeed when /dev/pts is available.)

* gnu/services/virtualization.scm (%virtual-build-machine-operating-system)
[file-systems]: Add %BASE-FILE-SYSTEMS.

Change-Id: I7d12a4cb491e957bf55e6c5f9dd09c013473ca42

gnu/services/virtualization.scm

@@ -1192,10 +1192,11 @@ authpriv.*;auth.info                    /var/log/secure
     (bootloader (bootloader-configuration         ;unused
                  (bootloader grub-minimal-bootloader)
                  (targets '("/dev/null"))))
-    (file-systems (list (file-system              ;unused
+    (file-systems (cons (file-system              ;unused
                           (mount-point "/")
                           (device "none")
-                          (type "tmpfs"))))
+                          (type "tmpfs"))
+                        %base-file-systems))
     (users (cons (user-account
                   (name "offload")
                   (group "users")