Quick addition to --version to mention rustc

[killercup]

As discussed in #2447, rustup versions are getting closer to rustc
versions (because the rustup maintainers are working too much I'm sure).
This leads to confusion when running rustup --version while intending
to run rustc --version.

The change introduced here will print two additional (stderr) lines
after rustup's default --version output. The first mentions this
issue, and the second will print the output of rustc --version as a
hint (by trivially calling out to rustc, whatever binary that may be).

[killercup]

r? @kinnison

For your entertainment I want to also quote this snippet from my testing:

$ env RUSTUP_FORCE_ARG0=rustup $CARGO_TARGET_DIR/debug/rustup-init --version
   Compiling rustup v1.22.1 (/home/pascal/code/rust-lang/rustup)
    Finished dev [unoptimized + debuginfo] target(s) in 9.35s
rustup 1.22.1+103 (000079b11 2020-08-19) dirty 3 modifications
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustup 1.22.1 (1980-01-01)`

(Renaming the binary to rustup instead of setting the env var fixes this of course.)

[rbtcollins]

(Renaming the binary to rustup instead of setting the env var fixes this of course.)

That suggests to me that you're invoking the proxy rather than grabbing the current active toolchain and invoking rustc via it, which won't have this defect at all, and will be measurably faster on windows, where process creation isn't as cheap as it is on Unix systems.

[kinnison]

Functionalitywise this looks fine, it'll be worth ensuring in a test that this works though. Probably just a cli-misc test.

[killercup]

Test added. Don't know how granular you test these kinds of things, so I hope the one simple test case is fine. I can always add more tests for edge cases if you want :)

[kinnison]

I think this is good enough for now. If we get unusual bug reports for --version we'll revisit more testing.