Since commit 6f36d0c89e3bf19bb7bb8f7f5b77de30c9af861b, Meson no longer
propagates Python. GTK+ propagates librsvg-bootstrap which causes a startup
error due to two librsvg being in GI_TYPELIB_PATH.
* gnu/packages/gnome.scm (piper)[inputs]: Add PYTHON. Remove LIBRSVG. Simplify.
[native-inputs]: Simplify.
[arguments]: Remove trailing #t from phases.