ci: run GIT_TEST_PACKED_REFS_VERSION=2 in some builds

The linux-TEST-vars CI build helps us check that certain opt-in features
are still exercised in at least one environment. The new
GIT_TEST_PACKED_REFS_VERSION environment variable now passes the test
suite when set to "2", so add this to that list of variables.

This provides nearly the same coverage of the v2 format as we had in the
v1 format.

Signed-off-by: Derrick Stolee <[email protected]>

ci/run-build-and-tests.sh

@@ -30,6 +30,7 @@ linux-TEST-vars)
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
 	export GIT_TEST_WRITE_REV_INDEX=1
 	export GIT_TEST_CHECKOUT_WORKERS=2
+	export GIT_TEST_PACKED_REFS_VERSION=2
 	;;
 linux-clang)
 	export GIT_TEST_DEFAULT_HASH=sha1