meson: Don't override RUSTFLAGS in the env

Meson does not add RUSTFLAGS to rustc.cmd_array(), so the effect of this code is to override that value in the env.

This is hacky, since the env has to match during setup and compile now, and rust_args added in the cross / native file will be ignored. But at least it fixes this regression:

Fixes #372 (closed)

Merge request reports