after stage0 redesign, x check defaults to building the compiler

[jyn514]

Summary

since #119899, x check has a global default of --stage 1. that makes builds for the default and compiler profiles much slower, because they are building the compiler before checking std.

we should do the following things:

• profile = "compiler" should set check-stage = 0 (this should be an easy change in defaults/compiler.toml)
• maybe the global default should also do that?
• x check --stage 0 std should give a warning if it doesn't already

note that after my suggested change, x check without arguments will no longer check the standard library for compiler profile. that seems ~fine.

reported in #t-infra/bootstrap > Stage 0 std redesign support thread @ 💬.

Command used

./x.py check

Expected behaviour

x check is a ~fast operation, like before.

Actual behaviour

x check takes about twice as long (because it defaults to --stage 1, which builds the compiler)

Bootstrap configuration (bootstrap.toml)

cp config.example.toml config.toml

Operating system

HEAD

52882f6

Additional context

Build Log

<log>