bff8a7a80d subprocess: replace __USING_WINDOWS__ with WIN32 (kevkevinpal)
Pull request description:
## Summary
Motivated by https://github.com/bitcoin/bitcoin/pull/34385#pullrequestreview-3826616188
In `subprocess.h` we are now renaming `__USING_WINDOWS__` with `WIN32`
In the rest of the codebase, we are using `WIN32`, so it makes sense to update `subprocess.h` to match that.
---
Use the following `grep` to assert there is no `__USING_WINDOWS__` in the codebase
```
grep -nri --exclude-dir=build "WIN32" ./ -I
rep -nri --exclude-dir=build "__USING_WINDOWS__" ./ -I
```
ACKs for top commit:
sedited:
ACK bff8a7a80d
hebasto:
ACK bff8a7a80d, I have reviewed the code and it looks OK.
Tree-SHA512: 18c3c8b87cf880053bbf69f837a0a135c5da51cfb15ab1d9fd554d8f931b2ea8202cf0f4d5e6f317d6234480128c2f41a7a1a9d9bd0504697a3c4c6a21797762