emsdk_env.sh doesn't work on cygwin due to wrong PATH parsing

the `emsdk_env.sh` script produces a jumbled up `PATH` when used under cygwin.

I was able to track the problem down to the wrong `ENVPATH_SEPERATOR` value beeing picked in `emsdk.py` (line 126-129).
The standard cygwin shell is bash so it uses ':' instead of ';' to seperate it's `PATH`, but here it is parsed as if it were using ';'.

I'm not sure how to fix it, I added an extra `and not MSYS` to the `if WINDOWS:` line(line 126), and that seemed to work, but I'm not sure if that's the optimal solution.

I'm also not sure what other things might me affected by this problem, everything else besides the mentioned `emsdk_env.sh` worked for me.