Fix ESP8266SdFat architecture, Windows CI (#7866)

* Fix ESP8266SdFat architecture

Avoid problems reported in
https://forum.arduino.cc/index.php?topic=726897.msg4889319

* Fix Windows CI, python3 now *maybe* exists

Python3 used to be called "python.exe" on earlier VMs, but it looks like
the image has been updated and a "python3.exe" does now exist.  Update
the CI script to first check it "python3" exists, and if not then do the
copy hack, OTW do nothing.

Author: earlephilhower

.github/workflows/pull-request.yml

@@ -102,7 +102,7 @@ jobs:
       run: |
         # Windows has python3 already installed, but it's called "python".
         # Copy python.exe to the proper name so scripts "just work".
-        copy (get-command python).source (get-command python).source.Replace("python.exe", "python3.exe")
+        try { Get-Command python3 } catch { copy (get-command python).source (get-command python).source.Replace("python.exe", "python3.exe") }
         bash ./tests/build.sh