Selects a rust-toolchain declared RA based on its date. The earliest (oldest) RA wins and becomes the one that the workspace uses as a whole.
In terms of precedence:
nightly > stable-with-version > stable
With stable-with-version, we invoke the RA with a `--version` arg and attempt to extract a date. Given the same date as a nightly, the nightly RA will win.
