gnu: librsvg-next: Update to 2.48.8.

gnu: librsvg-next: Update to 2.48.8.

* gnu/packages/gnome.scm (librsvg-next): Update to 2.48.8.
[source]: Remove substitute* from snippet.
[arguments]: In cargo-inputs replace rust-cairo-rs-0.7 with 0.8,
rust-cssparser-0.25 with 0.27, rust-float-cmp-0.5 with 0.6,
rust-gdk-pixbuf-0.7 with 0.8, rust-gio-0.7 with 0.8, rust-glib-0.8 with
0.9, rust-markup5ever-0.9 with 0.10, rust-nalgebra-0.18 with 0.19,
rust-pango-0.7 with 0.8, rust-pangocairo-0.8 with 0.9. Add rust-cast-0.2,
rust-once-cell-1.2, rust-rgb-0.8, rust-xml5ever-0.16. Remove
rust-lazy-static-1, rust-phf-0.7, rust-string-cache-0.7, rust-xml-rs-0.8.
In cargo-development-inputs replace rust-cairo-rs-0.7 with 0.8,
rust-criterion-0.2 with 0.3. In custom 'prepare-for-build phase use
cc-for-target. Remove 'remove-failing-tests phase.
[inputs]: Replace pango-1.42 with pango.