ma sources: refactor redundant musnix in [vw]bob, studio

1systems/vbob/source.nix

@@ -1,7 +1,4 @@
 import <stockholm/makefu/source.nix> {
-  name="studio";
-  override.musnix.git = {
-    url = https://github.com/musnix/musnix.git;
-    ref = "d8b989f";
-  };
+  name="vbob";
+  musnix = true;
 }

1systems/wbob/source.nix

@@ -1,7 +1,4 @@
 import <stockholm/makefu/source.nix> {
-  name="studio";
-  override.musnix.git = {
-    url = https://github.com/musnix/musnix.git;
-    ref = "d8b989f";
-  };
+  name="wbob";
+  musnix = true;
 }

source.nix

@@ -1,10 +1,17 @@
 with import <stockholm/lib>;
-host@{ name, secure ? false, override ? {}, full ? false, torrent ? false }: let
+host@{ name,
+  override ? {},
+  secure ? false,
+  full ? false,
+  torrent ? false,
+  musnix ? false
+}:
+let
   builder = if getEnv "dummy_secrets" == "true"
               then "buildbot"
               else "makefu";
   _file = <stockholm> + "/makefu/1systems/${name}/source.nix";
-  ref = "0751450"; # unstable @ 2017-07-16 + graceful requests2 (a772c3aa)
+  ref = "74b40c3"; # unstable @ 2017-07-16 + graceful requests2 (a772c3aa) + libpurple bitlbee
 
 in
   evalSource (toString _file) [
@@ -32,6 +39,12 @@ in
 
       stockholm.file = toString <stockholm>;
     }
+    (mkIf ( musnix ) {
+      musnix.git = {
+        url = https://github.com/musnix/musnix.git;
+        ref = "d8b989f";
+      };
+    })
     (mkIf ( torrent ) {
       torrent-secrets.file = getAttr builder {
         buildbot = toString <stockholm/makefu/6tests/data/secrets>;