Make this test unsupported when there is no real 'env' from a real shell.
While the test was already requiring shell-preserves-root (such as the
internal shell), it wasn't aware that the MSYS 1.0 env command knows how
to expand root by itself!

From cmd.exe try:

  env SDKROOT=/ cmd //c echo %SDKROOT%

And get:

  C:/MINGW/MSYS/1.0

To be certain we have a good 'env' program the test now requires a shell.