package/go: bump to 1.13.6
go1.13.6 (released 2020/01/09) includes fixes to the runtime and the net/http
package.
https://github.com/golang/go/issues?q=milestone=Go1.13.6
Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>