gnu: emacs-exwm: Provide absolute file name in '.desktop' file.

Previously GDM would crash when selecting EXWM.

* gnu/packages/emacs-xyz.scm (emacs-exwm)[arguments]: Rewrite
  'install-xsession' phase such that .desktop file contains an absolute file
  name.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/emacs-xyz.scm

@@ -7005,7 +7005,7 @@ (define-public emacs-exwm
                      Name=~a~@
                      Comment=~a~@
                      Exec=~a~@
-                     TryExec=~@*~a~@
+                     TryExec=~:*~a~@
                      Type=Application~%" ,name ,synopsis exwm-executable)))
                ;; Add a shell wrapper to bin
                (with-output-to-file exwm-executable