cmake(windows): set correct path to the system Git config

cmake(windows): set correct path to the system Git config

Currently, when Git for Windows is built with CMake, the system Git config is
expected in a different location than when building via `make`: the former
expects it to be in `<runtime-prefix>/mingw64/etc/gitconfig`, the latter in
`<runtime-prefix>/etc/gitconfig`.

Because of this, things like `git clone` do not work correctly (because cURL is
no longer able to find its certificate bundle that it needs to validate HTTPS
certificates). See the full bug report and discussion here:
https://github.com/git-for-windows/git/issues/3071#issuecomment-789261386.

This commit aligns the CMake-based build by mimicking what is already done in
`config.mak.uname`.

This closes https://github.com/git-for-windows/git/issues/3071.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>