Finishing Windows support?

[workingjubilee]

Thanks to the efforts of @usamoi, @passcod, and many bug reports and patches from others besides, we are within (slightly speculatively?) about 10 PRs of fixing Windows. Some of these may be very small patches.

Currently, cargo-pgrx builds on Windows, but does not quite finish cargo pgrx init:

   0: %1 is not a valid Win32 application. (os error 193)

Location:
   cargo-pgrx\src\command\init.rs:450

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SPANTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

   0: cargo_pgrx::command::init::download_postgres with pg_version=12.20
      at cargo-pgrx\src\command\init.rs:237

We probably mostly need to fix most of the shellouts and some of the path searches in the build system elements of pgrx so that they are more platform-neutral. This should mostly not require specialist knowledge (relative to "knowing a smidge of Rust", anyways).