Prepend the build root to the recipe dir in the vm

The build root is always '/', so that this change seems
to not do anything. But this fixes the

    if test "$MYSRCDIR" = $BUILD_ROOT/.build-srcdir; then

lines in the recipe implementions.

build

@@ -1018,7 +1018,7 @@ shopt -s nullglob
 export PATH=$BUILD_DIR:/sbin:/usr/sbin:/bin:/usr/bin:$PATH
 
 if vm_detect_2nd_stage ; then
-    set "/.build-srcdir/$RECIPEFILE"
+    set "$BUILD_ROOT/.build-srcdir/$RECIPEFILE"
     export PATH=/.build:$PATH
 fi