bump up runc

Changes: opencontainers/runc@96ec217...12f6a99

Including critical security fix for `runc run --no-pivot` (`DOCKER_RAMDISK=1`): opencontainers/runc#1962

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
(cherry picked from commit 3aec9e7)

vendor.conf

@@ -20,7 +20,7 @@ github.com/gogo/protobuf v1.0.0
 github.com/gogo/googleapis 08a7655d27152912db7aaf4f983275eaf8d128ef
 github.com/golang/protobuf 1643683e1b54a9e88ad26d98f81400c8c9d9f4f9
 github.com/opencontainers/runtime-spec v1.0.1
-github.com/opencontainers/runc 96ec2177ae841256168fcf76954f7177af9446eb
+github.com/opencontainers/runc 12f6a991201fdb8f82579582d5e00e28fba06d0a
 github.com/sirupsen/logrus v1.0.0
 github.com/pmezard/go-difflib v1.0.0
 github.com/urfave/cli 7bc6a0acffa589f415f88aca16cc1de5ffd66f9c