runInLinuxVM: load stdenv/setup with fixed environment in stage2Init

In [1] we started sourcing stdenv/setup in stage2Init to allow for
structuredAttrs. We failed to take the changed NIX_BUILD_TOP etc.
variables into account. We need to load stdenv/setup after changing
them, because the structuredAttrs startup code makes use of it.

[1]: 97ed6b4

pkgs/build-support/vm/default.nix

@@ -169,14 +169,14 @@ rec {
     if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
       source "$NIX_ATTRS_SH_FILE"
     fi
-    source $stdenv/setup
 
     export NIX_STORE=${storeDir}
     export NIX_BUILD_TOP=/tmp
     export TMPDIR=/tmp
     export PATH=/empty
     cd "$NIX_BUILD_TOP"
 
+    source $stdenv/setup
     if ! test -e /bin/sh; then
       ${coreutils}/bin/mkdir -p /bin
       ${coreutils}/bin/ln -s ${bash}/bin/sh /bin/sh