build: Build image should use Go 1.13 for now

Dockerfile.builder

@@ -1,5 +1,5 @@
 # We will grab the Go compiler from the latest Go image.
-FROM golang as go
+FROM golang:1.13 as go
 
 # Otherwise we base on the snapcraft container as that is by far the
 # most complex and tricky thing to get installed and working...