prefix echo and printf with env in test/spawn.jl

This change allows this test to pass on Windows using just the cut-down version of coreutils
that we currently bundle as part of msysgit, where echo and printf are shell scripts which
cannot be properly spawned. Slightly hacky, and runs into some newline differences at

```run(`env echo hello world` |> file)```

which results in a CRLF terminated file if you run it in MSYS2 or standalone,
but just LF if run inside Cygwin.

This approach will need to be revisited when we stop bundling msysgit.

Travis says this works on Linux, can someone test on Mac?