build: default PYTHON to python3 in Makefile

So that direct calls to `make` which spawn the configure script do not use Python 2.

PR-URL: #37764
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Christian Clauss <[email protected]>
Reviewed-By: Jiawen Geng <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ash Cripps <[email protected]>
Reviewed-By: Richard Lau <[email protected]>

Makefile

@@ -1,7 +1,7 @@
 -include config.mk
 
 BUILDTYPE ?= Release
-PYTHON ?= python
+PYTHON ?= python3
 DESTDIR ?=
 SIGN ?=
 PREFIX ?= /usr/local