[x.py] Option to not rebuild compiler #46180
Description
One of the biggest paper cuts when working on Rust documentation is that x.py is really really eager to rebuild the compiler, even when the only thing that is changing one line in documentation or update a test.
As such, it would be extremely beneficial to those writing docs and tests (including doctests!) if we could have a flag where we as the end user of the flag promise we're not changing actual compiler code, so please use the previous still good compiler to rebuild the docs with our changes. Note that the option doesn't have to actually rebuild the documentation, as ideally we'd just combine it with x.py doc and x.py test src/libstd as needed.
cc @rust-lang/infra