Determine rust channel by parsing rustc output if env vars do not exist (#163)

* Determine rust channel by parsing rustc output

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".

* Check env vars first, fall back to rustc in $PATH

* Try to check RUSTUP_TOOLCHAIN first

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