ci: use python3 in linux-gcc and osx-gcc and python2 elsewhere

Python2 reached end of life, and we have been preparing git-p4
(which is the sole in-tree user of Python) to work with Python3
but we haven't told our CI jobs to use Python3 anywhere yet.

Pick two jobs and use Python3 there, while leaving other jobs
to still stick to Python2 for now.

Signed-off-by: Junio C Hamano <[email protected]>

Author: szeder

ci/lib.sh

@@ -162,6 +162,9 @@ linux-clang|linux-gcc)
 	if [ "$jobname" = linux-gcc ]
 	then
 		export CC=gcc-8
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+	else
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 	fi
 
 	export GIT_TEST_HTTPD=true
@@ -182,6 +185,9 @@ osx-clang|osx-gcc)
 	if [ "$jobname" = osx-gcc ]
 	then
 		export CC=gcc-9
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+	else
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 	fi
 
 	# t9810 occasionally fails on Travis CI OS X