Determine rust channel by parsing rustc output if env vars do not exist

[itsCryne]

The RUSTUP_TOOLCHAIN environment variable might not always be present. This is the case for e.g. NixOS where rust is routinely not installed via rustup, thus not setting this env var, causing build failures. Instead, build.rs will now run rustc -V and check if the output contains the word "nightly".

[EightFactorial]

The copy of rustc in your path, if there even is one, isn't necessarily the version being used to build the project.

Maybe use RUSTUP_TOOLCHAIN or fall back to the RUSTC environment variable?

[itsCryne]

This will now also give an error if the beta channel is used

[itsCryne]

Is there anything left to do before this can get reviewed/merged?