Substitute all emacsclient occurrences in emacsclient.desktop

Substitute all emacsclient occurrences in emacsclient.desktop

After commit "From .desktop files, reuse a frame or start a new
Emacs as required" on 2021-06-30, there are two emacsclient
occurrences for the Exec key of etc/emacsclient.desktop.
Before this change, only the first occurrence is substituted.

* Makefile.in (install-etc): Substitute all emacsclient
occurrences in emacsclient.desktop.  (Bug#68803)

Copyright-paperwork-exempt: yes
(cherry picked from commit a73a1e00523c55942b953b55c3baaeb335e07e45)