Don't build Miri tool by default

[Noratrieb]

When setting extended = true in the config.toml, bootstrap also builds all the tools, including Miri. Since Miri doesn't always build, this can cause problems like in #100134. There is another option to only build a set of tools using tools = [] (this doesn't contain "miri" in the example form, which is a little odd, it should probably be added there as well).

rust-demangler is already excluded by default (unless profiling is enabled), so I think it would make sense to disable Miri by default as well (unless there's some good reason to not do it).

[Noratrieb]

@rustbot label +A-bootstrap

[RalfJung]

This is already implemented for stable/beta builds:

rust/src/bootstrap/tool.rs

Lines 807 to 809 in 3395094

	// By default, on nightly/dev enable all tools, else only
	// build stable tools.
	$stable || builder.build.unstable_features(),

I guess we could extend this to all builds, but then I am not sure if CI would still build these tools.

[jyn514]

I don't think this is the right fix, see #100134 (comment).