ci: Clarify --destdir help in install-common-dependencies

The --destdir option in install-meson-project is now cumulative,
so adjust the help output to clarify that.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4075>

.gitlab-ci/install-common-dependencies.sh

@@ -10,7 +10,8 @@ usage() {
 
 	Options:
 	  --libdir=DIR     Setup the projects with a different libdir
-	  --destdir=DIR    Install the projects to an additional destdir
+	  --destdir=DIR    Install the project to DIR, can be used
+	                   several times to install to multiple destdirs
 
 	  -h, --help       Display this help