When running `cargo test --release`, the artifacts from `buildPhase`
will be reused here. Previously, most of the stuff had to be recompiled
without optimizations.

This is done by gathering all binaries to install before running the
checkPhase.

Hydra nixpkgs: ?compare=1586582

There are several tarballs (such as the `rust-lang/rust`-source) with a
`Cargo.toml` at root and several sub-packages (with their own Cargo.toml)
without using workspaces[1].

In such a case it's needed to move into a subdir to only build the
specified sub-package (e.g. `rustfmt` or `rsl`), however the artifacts
are at `/target` in the root-dir of the build environment. This breaks
the build since `buildRustPackage` searches for executables in `target`
(which is at the build-env's root) at the end of the `buildPhase`.

With the optional `buildAndTestSubdir`-argument, the builder moves into
the specified subdir using `pushd`/`popd` during `buildPhase` and
`checkPhase`.

Also moved the logic to find executables and libs to the end of the `buildPhase`
from a custom `postBuild`-hook to fix packages with custom `build`/`install`-procedures
such as `uutils-coreutils`.

[1] https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html

A lot of tests are using `debug_assert!` which isn't available in
release-mode.

When running the default builder for Rust, the artifacts would be stored
in `target/<arch>/<profile>`, however the `install`-target expects the
default structure (`target/<profile>`) of `cargo`-builds.

When using the Makefile for building as well, the expected structure is
created instead.

Needed since we store artifacts in `target/<arch>/<profile>`.

See also https://discourse.nixos.org/t/rust-build-speed-improvements/7225

Co-authored-by: cole-h <cole.e.helbling@outlook.com>
Co-authored-by: asymmetric <lorenzo@mailbox.org>

...for rust-packages (into staging)