gnu: librewolf: Add %u to Exec option to open URLs.

The context behind this change is that Firefox used to ship a
taskcluster/docker/firefox-snap/firefox.desktop file which had an Exec
line like this:

    Exec=@MOZ_APP_NAME@ %u

The Guix package would use that file, replacing the token with the path
to the binary. Reported in #74648.

* gnu/packages/librewolf.scm (librewolf): Add %u to Exec option to open
URLs.

Change-Id: I8cf5d3886eaf7805209cf12eae0cc875bef6d5dd
Reviewed-by: André Batista <nandre@riseup.net>
Reviewed-by: Ian Eure <ian@retrospec.tv>
Signed-off-by: Sharlatan Hellseher <sharlatanus@gmail.com>

gnu/packages/librewolf.scm

@@ -605,7 +605,7 @@
                          (substitute* desktop-file
                            (("^Exec=@MOZ_APP_NAME@")
                             (string-append "Exec="
-                                           #$output "/bin/librewolf"))
+                                           #$output "/bin/librewolf %u"))
                            (("@MOZ_APP_DISPLAYNAME@")
                             "LibreWolf")
                            (("@MOZ_APP_REMOTINGNAME@")