fix drone tests failing on pre-mod Go versions.

.drone.jsonnet

@@ -7,7 +7,7 @@ local Pipeline(version, mod) = {
       name: "test",
       image: "golang:" + version,
       commands: [
-        if mod then "go mod download" else "go get",
+        if mod then "go mod download" else "go get -t",
         "go test -v ./...",
         "go test -bench ./..."
       ]
@@ -23,4 +23,4 @@ local Pipeline(version, mod) = {
   Pipeline("1.10", false),
   Pipeline("1.9", false),
   Pipeline("1.8", false),
-]
+]