vm/windows: Save envirenoment before running VM.

Later, when we start the actual builder, we're going to restore those
environment variables. We're using "(set; declare -p)", here, because
the former is just printing _all_ environment variables, even those not
supported, and the latter only lists specifically declared variables,
which also encludes exports.

The "declare -p" command also emits those variables in a format similar
to the "export" command.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/vm/windows/controller/default.nix

@@ -172,7 +172,9 @@ let
   });
 
   preVM = ''
+    (set; declare -p) > saved-env
     XCHG_DIR="$(${coreutils}/bin/mktemp -d nix-vm.XXXXXXXXXX --tmpdir)"
+    ${coreutils}/bin/mv saved-env "$XCHG_DIR/"
     QEMU_VDE_SOCKET="$(pwd)/vde.ctl"
     MONITOR_SOCKET="$(pwd)/monitor"
     ${vde2}/bin/vde_switch -s "$QEMU_VDE_SOCKET" &