Windows home_dir fallback when HOME is not set not supported

[RalfJung]

  0.159555   error: unsupported operation: can't call foreign function `GetUserProfileDirectoryW` on OS `windows`
  0.000051      --> /home/runner/work/miri-test-libstd/miri-test-libstd/rust-src-patched/library/std/src/sys/pal/windows/os.rs:329:23
  0.000008       |
  0.000008   329 |                   match c::GetUserProfileDirectoryW(
  0.000005       |  _______________________^
  0.000006   330 | |                     ptr::without_provenance_mut(CURRENT_PROCESS_TOKEN),
  0.000006   331 | |                     buf,
  0.000005   332 | |                     &mut sz,
  0.000009   333 | |                 ) {
  0.000010       | |_________________^ can't call foreign function `GetUserProfileDirectoryW` on OS `windows`