packer: fix build scripts, packerlib update.

There were some hardcoded values left in there, and the remote caching
wasn't working right.

The ubuntu-board-gen tool now produces the right output file
name.

packer/packerlib/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "packerlib",
@@ -13,3 +13,9 @@ go_library(
     visibility = ["//visibility:public"],
     deps = ["@in_gopkg_yaml_v2//:yaml_v2"],
 )
+
+go_test(
+    name = "packerlib_test",
+    srcs = ["ubuntu_test.go"],
+    embed = [":packerlib"],
+)

packer/packerlib/ubuntu.go

@@ -61,7 +61,6 @@ func (spec UbuntuBoardSpec) JSONPath(base string) string {
 	dest := spec.ImageName
 	ext := filepath.Ext(dest)
 	if ext != "" {
-		ext = "." + ext
 		dest = strings.TrimSuffix(dest, ext)
 	}