Move distributed std's profile settings to its workspace

[adamgemmell]

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The configuration used by std needs to be moved to somewhere the new build-std can see it. This PR moves settings that Cargo can model in its profile to the dist profile and ensures that bootstrap does not try to override them.

This PR adds a new Mode, DistStd, to bootstrap, as proposed by @Kobzol a while ago, which is used when building the standard library during x dist builds. This mode:

• Uses the dist profile in the library workspace, added in Move bootstrap configuration to library workspace #149514
  • Normal builds revert back to using release/dev, which fixes building std without optimisations
• Disallows overriding any profile fields. Configuration in dist builds that do this should instead place the configuration in the library workspace so that it can also be used by build-std.

Note that RUSTFLAGS can still be set based on configuration, and this will be addressed in an upcoming patch as we still need to work out how build-std can consume target-specific configuration. Probably this will go in the cargo config.