Update to go 1.15.8

go1.15.8 (released 2021/02/04) includes fixes to the compiler, linker, runtime,
the go command, and the net/http package. See the Go 1.15.8 milestone on the
issue tracker for details.

https://github.com/golang/go/issues?q=milestone%3AGo1.15.8+label%3ACherryPickApproved

full diff: https://github.com/golang/go/compare/go1.15.7...go1.15.8

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

.zuul/playbooks/containerd-build/run.yaml

@@ -2,7 +2,7 @@
   become: yes
   roles:
   - role: config-golang
-    go_version: '1.15.7'
+    go_version: '1.15.8'
     arch: arm64
   tasks:
   - name: Build containerd