Include build instructions for setting path to Rust toolchain

[guest271314]

I have Rust on a USB, not installed on the operating system proper. How do I tell cmake or make where to find the Rust toolchain?

make 
# ...
info: rolling back changes
CMake Error at deps/cpm_cache/corrosion/f16c/cmake/FindRust.cmake:23 (message):
  Could not find toolchain '1.88.0'

  Available toolchains:

Call Stack (most recent call first):
  deps/cpm_cache/corrosion/f16c/cmake/FindRust.cmake:468 (_findrust_failed)
  deps/cpm_cache/corrosion/f16c/cmake/Corrosion.cmake:63 (find_package)
  deps/cpm_cache/corrosion/f16c/CMakeLists.txt:73 (include)


-- Configuring incomplete, errors occurred!

[tschneidereit]

I think this might be possible using env vars, but I honestly don't know. I'm afraid I also don't have the time to dig into it, but if you find out how to do this, a PR with a short addition to the build instructions would be welcome.

[guest271314]

I'll see what happens if I decide to dig in to StarlingMonkey a little more, which I probably will.