CI: use "$runs_on_pool", not "$jobname" to select packages & config

Change the setup hooks for the CI to use "$runs_on_pool" for the
"$regular" job. Now we won't need as much boilerplate when adding new
jobs to the "regular" matrix, see 956d2e4639 (tests: add a test mode
for SANITIZE=leak, run it in CI, 2021-09-23) for the last such commit.

I.e. now instead of needing to enumerate each jobname when we select
packages we can install things depending on the pool we're running
in.

That we didn't do this dates back to the now gone dependency on Travis
CI, but even if we add a new CI target in the future this'll be easier
to port over, since we can probably treat "ubuntu-latest" as a
stand-in for some recent Linux that can run "apt" commands.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -156,11 +156,15 @@ export DEFAULT_TEST_TARGET=prove
 export GIT_TEST_CLONE_2GB=true
 export SKIP_DASHED_BUILT_INS=YesPlease
 
-case "$jobname" in
-linux-clang|linux-gcc|linux-leaks)
+case "$runs_on_pool" in
+ubuntu-latest)
+	if test "$jobname" = "linux-gcc-default"
+	then
+		break
+	fi
+
 	if [ "$jobname" = linux-gcc ]
 	then
-		export CC=gcc-8
 		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python3"
 	else
 		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python2"
@@ -180,17 +184,17 @@ linux-clang|linux-gcc|linux-leaks)
 	GIT_LFS_PATH="$HOME/custom/git-lfs"
 	export PATH="$GIT_LFS_PATH:$P4_PATH:$PATH"
 	;;
-osx-clang|osx-gcc)
+macos-latest)
 	if [ "$jobname" = osx-gcc ]
 	then
-		export CC=gcc-9
 		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
 	else
 		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
 	fi
 	;;
-linux-gcc-default)
-	;;
+esac
+
+case "$jobname" in
 linux32)
 	CC=gcc
 	;;
@@ -200,9 +204,6 @@ linux-musl)
 	MAKEFLAGS="$MAKEFLAGS NO_REGEX=Yes ICONV_OMITS_BOM=Yes"
 	MAKEFLAGS="$MAKEFLAGS GIT_TEST_UTF8_LOCALE=C.UTF-8"
 	;;
-esac
-
-case "$jobname" in
 linux-leaks)
 	export SANITIZE=leak
 	export GIT_TEST_PASSING_SANITIZE_LEAK=true