Cannot run any run-make test without checking out the cargo submodule

[RalfJung]

I generally want to have as few git submodule checked out as possible in the rustc repo, since all the checked out submodules need to be synced each time they change and I'd like to minimize the number of time I have to run git submodule update --recursive. Therefore, I do not have the cargo submodule checked out.

However, this means I cannot run ./x test run-make. My understanding is that some run-make tests need cargo, but most do not, so it is unfortunate that I cannot run the non-cargo-using run-make tests without checking out cargo. Would be nice to check this in the test rather than blanket rejecting the entire test suite when cargo is missing.

(Also maybe one could use bootstrap cargo instead of the submodule? But I assume there's a reason the submodule is used so this probably does not work.)

Cc @jieyouxu

[jieyouxu]

• The some run-make needing cargo/rustdoc tests part is Split run-make into two test suites: fast-path that don't need to build cargo and a slow-path that builds cargo #134109. I just haven't gotten to this yet because I want to kill the Makefiles first.
• Needing to build a suitably-staged cargo is because Pass the current cargo to run-make tests #130642

A couple tests were using BOOTSTRAP_CARGO with -Zbuild-std, but that
stage0 cargo might not always be in sync with in-tree changes. In
particular, those tests started failing on the beta branch because the
older cargo couldn't find the library Cargo.lock, and then couldn't
build the latest version of compiler_builtins that had nightly changes.

[jieyouxu]

I'm going to close this in favor of #134109, but I do appreciate that this is annoying, and is something I'd like to look into towards end of this month or early next month.