fix(shell): Use a distinct style for transient status

PR rust-lang#15928 made `Checking` and `Building` the same style because the
transient state is `note:` and rust-lang#15928 made `note:~ the same as our
regular headers.

This switches the transient state to `help:` until we can further
explore color choices with rustc.

See also https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/colors.20removed.20in.201.2E91.20.28current.20beta.29.3F/near/541900531

Author: epage

src/cargo/util/style.rs

@@ -11,4 +11,4 @@ pub const NOTE: Style = annotate_snippets::renderer::DEFAULT_NOTE_STYLE;
 pub const GOOD: Style = AnsiColor::BrightGreen.on_default().effects(Effects::BOLD);
 pub const VALID: Style = AnsiColor::BrightCyan.on_default().effects(Effects::BOLD);
 pub const INVALID: Style = annotate_snippets::renderer::DEFAULT_WARNING_STYLE;
-pub const TRANSIENT: Style = NOTE;
+pub const TRANSIENT: Style = annotate_snippets::renderer::DEFAULT_HELP_STYLE;