vcxproj: only copy `git-remote-http.exe` once it was built

vcxproj: only copy `git-remote-http.exe` once it was built

In b18ae14a8f6 (vcxproj: also link-or-copy builtins, 2019-07-29), we
started to copy or hard-link the built-ins as a post-build step of the
`git` project.

At the same time, we tried to copy or hard-link `git-remote-http.exe`,
but it is quite possible that it was not built at that time.

Let's move that latter task into a post-install step of the
`git-remote-http` project instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>