… the nightly channel

This was the whole point of rust-lang/rust#77802.

- Pass `pkg.name()` to `parse()`. This can't pass the `Package` directly
  because `PackageInner` is an `Rc` and therefore not thread-safe. Note
  that `pkg_name` was previously a *description* of the package, not the
  name passed with `--crate-name`.

Previously, since `ENABLE_NIGHTLY_FEATURES` and
`NIGHTLY_FEATURES_ENABLED` were thread locals, reading them in any other
thread would always say nightly features were disabled. Now, they are
tied to the `Context` itself, so it is both more clear how the variables
are being set and fixes the behavior within work threads.

Note that `Context` is not thread-safe, so this passes a boolean through
to `BuildOutput::parse`.

This avoids reparsing `channel()` over and over again. It also makes
`maybe_enable_nightly_features` unnecessary and removes it.

`nightly_features_allowed()` is no longer doing any work, so it can be
accessed directly. This also renames the `enable_nightly_features` field
to `nightly_features_allowed`.

Note that this has to be set in the builder, before
`config.configure()` gets run.

Co-authored-by: Eric Huss <eric@huss.org>

The info was already present on `self`.

Note that this uses a temporary variable to avoid a borrowck error from
`slot`.

It didn't have any information about RUSTC_BOOTSTRAP itself, only the
general `rustc-env` feature.