runInLinuxVM: re-add sourcing of stdenv & .attrs.sh

This is required for `preVM` which can be arbitrary code: e.g.
`vmTools.createEmptyImage` assumes the presence of `$out` in its default
configuration.

Also, before this patch, `preVM` wasn't even set in the
`__structuredAttrs` case.

pkgs/build-support/vm/default.nix

@@ -240,6 +240,10 @@ rec {
 
 
   vmRunCommand = qemuCommand: writeText "vm-run" ''
+    if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
+      source "$NIX_ATTRS_SH_FILE"
+    fi
+    source $stdenv/setup
     export > saved-env
 
     PATH=${coreutils}/bin