[windows] Add "C:\Program Files\Git\usr\bin\pinentry.exe" to windows pinentry discovery

[bendem]

Using vault pin set, I'm getting

> "$gw" vault pin set
Pin updating failed: No pinentry found, checked a bunch of different places

Pinentry is very commonly available from Git itself using C:\Program Files\Git\usr\bin\pinentry.exe. Might I suggest to add this path to https://github.com/quexten/goldwarden/blob/main/cli/agent/systemauth/pinentry/keybase-pinentry/pinentry_windows.go