What is go.exe, and why's a 32-bit x86 Windows binary checked into the repo?

See https://github.com/tailscale/tailscale/pull/19256

In summary, our previous attempts to provide a version of ./tool/go (a
shell script) on Windows with PowerShell and cmd.exe both were
lacking.

So now we we're regrettably checking in a binary to the tree. Its
source code is in ./tool/goexe. It's written in Rust without std so
it's very small (smaller than plenty of of our source code files!) and
it's 32-bit x86 so it runs on 32-bit x86, 64-bit x86, and arm64 Windows
where it's emulated.

This binary is not required, but it's used by our build system and
people working on Tailscale who are used to being able to run
"./tool/go" and have it do the right hermetic thing, using the correct
Go toolchain.