Bump RUST_STABLE to 1.60.0 to get -Cinstrument-coverage
Bump RUST_STABLE to 1.60.0 to get -Cinstrument-coverage

This does not raise the MSRV, just the day-to-day one.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/724>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg