cmake(install): fix double .exe suffixes

cmake(install): fix double .exe suffixes

By mistake, the `.exe` extension is appended _twice_ when installing the
dashed executables into `libexec/git-core/` on Windows (the extension is
already appended when adding items to the `git_links` list in the
`#Creating hardlinks` section).

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