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

@@ -15,7 +15,7 @@ case "$runs_on_pool" in
 ubuntu-*)
 	sudo apt-get -q update
 	sudo apt-get -q -y install language-pack-is libsvn-perl apache2 \
-		$UBUNTU_COMMON_PKGS $CC_PACKAGE
+		$UBUNTU_COMMON_PKGS $CC_PACKAGE $PYTHON_PACKAGE
 	mkdir --parents "$P4_PATH"
 	pushd "$P4_PATH"
 		wget --quiet "$P4WHENCE/bin.linux26x86_64/p4d"

ci/lib.sh

@@ -232,12 +232,12 @@ ubuntu-*)
 		break
 	fi
 
-	if [ "$jobname" = linux-gcc ]
+	PYTHON_PACKAGE=python2
+	if test "$jobname" = linux-gcc
 	then
-		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