Builder now always builds with nightly

[VZout]

This allows a lot more flexibility in using the builder in stable repositories.
Note that it doesn't install components.

[khyperia]

If we do go this route, could we add a test to make sure rust-toolchain and this string match? I can absolutely see this accidentally getting out of date.

For example, extract this string to a const, and write a test to make sure rust-toolchain contains the string.

[eddyb]

You can use include_str! + parsing the toml in a (spirv-builder/build.rs) build script, to compute the value, let Cargo record it, and then env!(...) to read it in spirv-builder/src/lib.rs (assuming we don't want to parse the TOML at runtime, but maybe a simple toolchain = " string search would work?).

But also we could bundle the rust-toolchain with the spirv-builder and drop it in the target directory, or in some temporary directory elsewhere, and use that directory as the working dir for the nested Cargo invocation.

[VZout]

Depends on #239