findrust: Simplify (#11328)

FindRust is too clever by half. It tries to do rustup's job for it.

See b38551dde9 for how that can break.

So we simplify it, and only let it check three things:

- Where's rustc? Look in $PATH and ~/.cargo/bin
- Where's cargo? Look in $PATH and ~/.cargo/bin
- What is the rust target (because we pass it explicitly)?

If any of these aren't that simple, we'll ask the user to tell us,
by setting Rust_COMPILER, Rust_CARGO or Rust_CARGO_TARGET.

None of the other things are helpful to us - we do not support windows
or whatever a "unikraft" is, and if the rust version doesn't work
it'll print its own error.

We could add a rustc version check, but that will become less and less
useful because rustc versions since 1.56 (released October 2021) will check rust-version in
Cargo.toml. So even at this point it's only pretty old rust versions already.

README.rst

@@ -159,6 +159,9 @@ CMake Build options
 
 In addition to the normal CMake build options (like ``CMAKE_INSTALL_PREFIX``), fish's CMake build has some other options available to customize it.
 
+- Rust_COMPILER=path - the path to rustc. If not set, cmake will check $PATH and ~/.cargo/bin
+- Rust_CARGO=path - the path to cargo. If not set, cmake will check $PATH and ~/.cargo/bin
+- Rust_CARGO_TARGET=target - the target to pass to cargo. Set this for cross-compilation.
 - BUILD_DOCS=ON|OFF - whether to build the documentation. This is automatically set to OFF when Sphinx isn't installed.
 - INSTALL_DOCS=ON|OFF - whether to install the docs. This is automatically set to on when BUILD_DOCS is or prebuilt documentation is available (like when building in-tree from a tarball).
 - FISH_USE_SYSTEM_PCRE2=ON|OFF - whether to use an installed pcre2. This is normally autodetected.