Auto merge of #142581 - Kobzol:bootstrap-std-method, r=jieyouxu

Enforce in bootstrap that build must have stage at least 1

This PR is a step towards https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Proposal.20to.20cleanup.20stages.20and.20steps.20after.20the.20redesign/with/523586917. It's very hard or me to make self-contained changes to bootstrap at this moment, so this PR kind of does several things:

1) (first two commits) Try to reduce the usage of `Std::new` in bootstrap, and replace it with a `Builder::std` method (similar to `Builder::compiler`). This is mostly to remove executions of the `Std` step for stage 0, which doesn't make a lot of sense; I'd like to ideally have the invariant that when a step is invoked, it actually builds or does something. Eventually, I'd like for everything to go through `Builder::std`. (Note: I'm not totally married to this idea, if you don't like it, we can remove it from this PR. I mostly did it right now to remove stage 0 std steps from snapshot tests, which shouldn't be there, but we can also filter them out in a different way)
2) Make sure that when you pass `x build compiler`, only the `Assemble` root level step will be invoked, and not the `Rustc` step. Before, both were invoked, which actually ran `Rustc` twice, once with all `crates` filled, and once with no crates (but both actually represent the same situation). Since the `Rustc::make_run` step actually requests a compile that is one stage below it, this actually made `build compiler --stage 0` work, which we don't want to have anymore.
3) Enforce a bootstrap-global invariant that all `build` commands are always on stage `>=1`. If you try to `build` anything on stage 0, it will print a warning and exit bootstrap. This follows the intuition from the new staging rules after the stage redesign; artifacts that are "stage 0" come outside of bootstrap, and we can't really build something for which we don't have source (although we can still test it, but that's for another PR).

Now the logic for build should be quite simple. For pretty much everything except for `Std`, you first use the stage0 compiler to build stage 1. Then you can build a stage 2 <something> using the previously built stage 1 (and then you can continue to stage 3 etc.). And that's it. The default build stage for everything is 1 (modulo download-ci-rustc, but that's a separate can of worms).

The snapshot test infra isn't super nice at the moment, as one of next steps I want to create some simple Builder pattern that will allow us to configure the bootstrap invocations in a more "forward-compatible" way (e.g. now it's not possible to modify the config passed to `configure_with_args`).

There are some things not yet fully resolved for build stage 0:
1) Cargo is still a `ModeRustc` tool, even though it doesn't really have to be, it is buildable with the stage0 compiler
2) bootstrap tools (`opt-dist`, `build-manifest` etc.) are still called stage0 tools, and in the bootstrap output it says something like "stage 0 rustc builds stage 0 opt-dist". Which is a bit weird, but functionally there's no difference, it's just a slightly inconsistent output. We still haven't decided if we should make these tools ignore staging altogether (which is IMO the right choice) or if we want to allow building stage 1/2/3/... bootstrap tools.

r? `@jieyouxu`

try-job: x86_64-rust-for-linux

Author: bors