Checks if we're in a nightly build.
The environment variable
CFG_RELEASE_CHANNEL is set during the rustc bootstrap
to "stable", "beta", or "nightly" depending on what toolchain is being built.
If we are being built as part of the stable or beta toolchains, we want
to disable unstable configuration options.
If we're being built by cargo (e.g.,
cargo +nightly install rustfmt-nightly),
CFG_RELEASE_CHANNEL is not set. As we only support being built against the
nightly compiler when installed from crates.io, default to nightly mode.