runInLinuxVM: refactor vmRunCommand

This makes it simpler to copy more files to xchg for the structuredAttrs
case in the next commit.

Author: wolfgangwalther

pkgs/build-support/vm/default.nix

@@ -230,16 +230,15 @@ rec {
 
 
   vmRunCommand = qemuCommand: writeText "vm-run" ''
-    export > saved-env
+    ${coreutils}/bin/mkdir xchg
+    export > xchg/saved-env
+    PATH=${coreutils}/bin
+
     if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
       source "$NIX_ATTRS_SH_FILE"
     fi
     source $stdenv/setup
 
-    PATH=${coreutils}/bin
-    mkdir xchg
-    mv saved-env xchg/
-
     eval "$preVM"
 
     if [ "$enableParallelBuilding" = 1 ]; then