contrib: use a command to clean the meson contrib before rebuilding

Now the meson build folder is handled in a single place.

(cherry picked from commit d0055d6c) (edited)
edited:
- 3.0 doesn't build libplacebo with meson
- 3.0 doesn't have basu
- 3.0 doesn't have librist
- 3.0 doesn't have medialibrary