update containerd binary to v1.3.8

full diff: https://github.com/containerd/containerd/compare/v1.3.7...v1.3.8

Release notes:

containerd 1.3.8
----------------------

Welcome to the v1.3.8 release of containerd!

The eighth patch release for containerd 1.3 includes several bug fixes and updates.

Notable Updates

- Fix metrics monitoring of v2 runtime tasks
- Fix nil pointer error when restoring checkpoint
- Fix devmapper device deletion on rollback
- Fix integer overflow on Windows
- Update seccomp default profile

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/containerd.installer

@@ -4,7 +4,7 @@
 # containerd is also pinned in vendor.conf. When updating the binary
 # version you may also need to update the vendor version to pick up bug
 # fixes or new APIs.
-: "${CONTAINERD_COMMIT:=8fba4e9a7d01810a393d5d25a3621dc101981175}" # v1.3.7
+: "${CONTAINERD_COMMIT:=7fb6e171309113ddcb8ea9599e34321550469250}" # v1.3.8
 
 install_containerd() {
 	echo "Install containerd version $CONTAINERD_COMMIT"