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

pkgs/development/tools/rust/rust-analyzer/generic.nix

@@ -15,9 +15,7 @@ rustPlatform.buildRustPackage {
     inherit rev sha256;
   };
 
-  preBuild = "pushd crates/rust-analyzer";
-  # Do not checking other crates in checkPhase.
-  preInstall = "popd";
+  buildAndTestSubdir = "crates/rust-analyzer";
 
   cargoBuildFlags = lib.optional useJemalloc "--features=jemalloc";