Windows: download busybox-win32 during make win-extras

copy it to echo.exe and printf.exe during make dist
This allows test/spawn.jl to succeed on Windows, since echo and printf
are only included as shell scripts in msysgit (and an echo.bat in
contrib/windows), but libuv cannot spawn shell or bat scripts.

This is an alternative to #7349. Use of busybox can be either removed
or expanded upon in the future when msysgit is no longer bundled with
Windows Julia, for now this is just to make the spawn tests pass.

cc @vtjnash
https://github.com/rmyorston/busybox-w32 for more info on busybox-win32 -
it's unfortunately GPLv2, but so's msysgit so no big difference there.
How about moving this and GIT to a new section "Julia bundles the following as external executables on some platforms: ..." to make a distinction between libraries we link against, and things we include and shell out to.