tests/int/cgroups: don't use BUSYBOX_BUNDLE
tests/int/cgroups: don't use BUSYBOX_BUNDLE

Commit 41670e21f removed BUSYBOX_BUNDLE env var, but c3ffd2ef81aa
was developed before 41670e21f was merged.

Everything still works because now BUSYBOX_BUNDLE has no value.
Nevertheless, let's remove it to avoid confusion.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

rhatdan/runc,dqminh/runc,dqminh/runc,kolyshkin/runc,opencontainers/runc,dqminh/runc,kolyshkin/runc,rhatdan/runc,rhatdan/runc,kolyshkin/runc,dqminh/runc,opencontainers/runc,opencontainers/runc,kolyshkin/runc,rhatdan/runc,opencontainers/runc