ci: install python on ubuntu

Python is missing from the default ubuntu-22.04 runner image, which
prevents git-p4 from working. To install python on ubuntu, we need
to provide the correct package names:

 * On Ubuntu 18.04 (bionic), "/usr/bin/python2" is provided by the
   "python" package, and "/usr/bin/python3" is provided by the "python3"
   package.

 * On Ubuntu 20.04 (focal) and above, "/usr/bin/python2" is provided by
   the "python2" package which has a different name from bionic, and
   "/usr/bin/python3" is provided by "python3".

Since the "ubuntu-latest" runner image has a higher version, its
safe to use "python2" or "python3" package name.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/install-dependencies.sh

@@ -16,7 +16,7 @@ linux-clang|linux-gcc)
 	sudo apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
 	sudo apt-get -q update
 	sudo apt-get -q -y install language-pack-is libsvn-perl apache2 \
-		$UBUNTU_COMMON_PKGS
+		$UBUNTU_COMMON_PKGS $PYTHON_PACKAGE
 	case "$jobname" in
 	linux-gcc)
 		sudo apt-get -q -y install gcc-8

ci/lib.sh

@@ -184,13 +184,13 @@ export SKIP_DASHED_BUILT_INS=YesPlease
 
 case "$jobname" in
 linux-clang|linux-gcc)
+	PYTHON_PACKAGE=python2
 	if [ "$jobname" = linux-gcc ]
 	then
 		export CC=gcc-8
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python3"
-	else
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python2"
+		PYTHON_PACKAGE=python3
 	fi
+	MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/$PYTHON_PACKAGE"
 
 	export GIT_TEST_HTTPD=true