ci: Improve install-meson-project usage output

Most of the options require a parameter, indicate that in the
usage output.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3497>

.gitlab-ci/install-meson-project.sh

@@ -10,10 +10,10 @@ usage() {
 
 	Options:
 	  -Dkey=val          Option to pass on to meson
-	  --subdir       Build subdirectory instead of whole project
-	  --prepare      Script to run before build
-	  --libdir       Setup the project with a different libdir
-	  --destdir      Install the project to an additional destdir
+	  --subdir=DIR       Build subdirectory instead of whole project
+	  --prepare=SCRIPT   Script to run before build
+	  --libdir=DIR       Setup the project with a different libdir
+	  --destdir=DIR      Install the project to an additional destdir
 
 	  -h, --help         Display this help