meson: fix installation when -Dlibexexdir is set

meson: fix installation when -Dlibexexdir is set

commit 837f637cf5 ("meson.build: correct setting of GIT_EXEC_PATH",
2025-05-19) corrected the GIT_EXEC_PATH build setting, but then forgot
to update the installation path for the library executables. This causes
a regression when attempting to execute commands, after installing to a
non-standard location (reported here[1]):

    $ meson -Dprefix=/tmp/git -Dlibexecdir=libexec-different build
    $ meson install
    $ /tmp/git/bin/git --exec-path
    /tmp/git/libexec-different
    $ /tmp/git/bin/git daemon
    git: 'daemon' is not a git command. See 'git --help'

In order to fix the issue, use the 'git_exec_path' variable (calculated
while processing -Dlibexecdir) as the 'install_dir' field during the
installation of the library executables.

[1]: <66fd343a-1351-4350-83eb-c797e47b7693@gmail.com>

Reported-by: irecca.kun@gmail.com
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>