bootstrap: explicitly request copying rustc-dev artifacts for rustfmt/clippy under download-rustc

[jieyouxu]

Summary

#t-infra/bootstrap > x run rustfmt works but x test rustfmt fails to compile reported that ./x test rustfmt --stage={1,2} no longer works with download-rustc.

• This seems to be due to us removing some manual builder.ensure(Rustc)s around the times of stage 0 rejiggling.
• Because compile::Sysroot::run has a quirky non-trivial behavior regarding rustc-dev artifact copying under download-rustc, tracing back to sysroot dep resolution troubles (cf. tests/ui/allocator/no_std-alloc-error-handler* fail when download-rustc is enabled #108767), we need to explicitly ensure(Rustc) to opt-in to rustc-dev artifact copying under download-rustc.
• For the short-term, let's insert manual ensures to force rustc-dev artifact copying under download-rustc to unblock rustfmt/clippy contributors who want to use download-rustc (to avoid having to build the compiler).

Testing

Use a dummy config: bootstrap.rustfmt.toml

profile = "tools"
change-id = "ignore"

[rust]
# Force download-rustc, needed because we modify bootstrap which makes
# `if-unchanged` ineligible.
download-rustc = true

Run:

• ./x test rustfmt --config=bootstrap.rustfmt.toml --stage=1
• ./x test rustfmt --config=bootstrap.rustfmt.toml --stage=2
• ./x test clippy --config=bootstrap.rustfmt.toml --stage=1
• ./x test clippy --config=bootstrap.rustfmt.toml --stage=2

When reviewing, also check that if you revert the commit / this PR, that invocations above fail (which is the case against main for me).

Tested also without download-rustc to make sure ./x test {rustfmt,clippy} --stage={1,2} still works.

Notes

This is by no means a proper fix, but it should unblock local tool profile workflows trying to use download-rustc with {rustfmt,clippy}. I don't think it's worth blocking over that.

Efforts for a more proper fix should be tracked by #156525, I left FIXMEs pointing to that.

[rustbot]

r? @Mark-Simulacrum

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