`home_dir` incorrectly considers `HOME` environment variable on Windows

[brson]

The definition of home_dir provided by the standard library is incorrect because it considers the HOME environment variable on Windows. This causes surprising situations where a Rust program will behave differently depending on whether it is run under a Unix emulation environment like Cygwin or MinGW. Neither Cargo nor rustup use the standard libraries definition - they use the definition here.

I don't see an existing issue for this.

[durka]

#28940 #40320

[Mark-Simulacrum]

Closing in favor of #28940.