Adapt project/make.sh for Windows builds

Fixes:
- link `-H windows` is not compatible with `-linkmode external`
- under Cygwin go does not play well with cygdrive type paths (i.e.: `/cygdrive/e/src/...`)

These issues were encountered while examining the required work for proper integration tests on client only platforms (#9217).
But do `=` and `==` operators behave differently within double-brackets? :smile: I'm not trying to argue here, I'm just trying to understand, but in doubt I'll just switch to `==`, thanks!