fix(ci): prune GOCACHE if it gets too big

scripts/prune-go-cache.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+
+set -o errexit
+set -o pipefail
+
+
+GOCACHE=$(go env GOCACHE)
+SIZE=`du -sxm ${GOCACHE} | awk '{print $1}'`
+echo "GOCACHE size: ${SIZE}MB"
+
+if [ $SIZE -gt 3500 ]; then
+  echo "Pruning GOCACHE at ${GOCACHE}"
+  find "${GOCACHE}" -type f -delete
+fi