ci: turn rustc warnings into errors in the CI
ci: turn rustc warnings into errors in the CI

followup of !431 [1]

deny(warnings) is an anti-pattern as a newer release of
the compiler can cause working/released builds to stop
compilling for downstreams.

Having to export a RUST_FLAG is not the prettiest, but
its the best we got atm [2]

[1] https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/431
[2] https://github.com/rust-lang/cargo/issues/3591

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