ci: Support setting libdir in install-meson-project.sh

ci: Support setting libdir in install-meson-project.sh

Besides building custom fedora images,  we will build
system extensions for GNOME OS. Since GNOME OS uses a
a different libdir compared to what meson defaults to,
we must allow setting libdir in this script.

See https://discourse.gnome.org/t/towards-a-better-way-to-hack-and-test-your-system-components/21075
See https://gitlab.gnome.org/GNOME/gnome-build-meta/-/issues/837

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3390>