Support selecting a different Rust toolchain

At the moment the values passed through the RUSTC and CARGO environment
variables when running configure are ignored and just `rustc`/`cargo`
as found on the PATH are used.

This is problematic during package builds where we might have to
select a specific Rust toolchain, i.e., make sure to always use the
system Rust toolchain vs a rustup toolchain.

Signed-off-by: Tobias Kortkamp <tobik@FreeBSD.org>
7 jobs for patch-use-configured-rust-toolchain in 34 minutes and 22 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #189415
fedora test

00:08:42

 
  Lint
failed #189417
allowed to fail manual
clippy

00:03:28

passed #189416
rustfmt

00:01:30

passed #189418
tarball

00:12:11

 
  Cross Distro
passed #189421
debian_i386:testing

00:09:58

passed #189420
debian:testing

00:08:59

passed #189419
fedora:rawhide

00:08:59

 
Name Stage Failure
failed
clippy Lint

Caused by:
editions are unstable

Caused by:
feature `edition` is required

consider adding `cargo-features = ["edition"]` to the manifest
ERROR: Job failed: exit code 1