rustPlatform: add `buildAndTestSubdir`-argument

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

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