rules_go stamping issue

bazel-test.log

$ bazel test //... --test_output=errors
INFO: Invocation ID: 44446d09-160e-4736-a899-168fd8b1e963
INFO: Analyzed 3 targets (0 packages loaded, 0 targets configured).
INFO: Found 2 targets and 1 test target...
FAIL: //:go_default_test (see /private/var/tmp/_bazel_robbert/df320eb0fd1e2eed12c0b1f31ac9635c/execroot/__main__/bazel-out/darwin-fastbuild/testlogs/go_default_test/test.log)
INFO: From Testing //:go_default_test:
==================== Test output for //:go_default_test:
--- FAIL: TestStamp (0.00s)
    main_test.go:7: compound = "unknown"; simple = "robbert"
FAIL
================================================================================
INFO: Elapsed time: 0.203s, Critical Path: 0.07s
INFO: 2 processes: 1 remote cache hit, 1 darwin-sandbox.
INFO: Build completed, 1 test FAILED, 2 total actions
//:go_default_test                                                       FAILED in 0.1s
  /private/var/tmp/_bazel_robbert/df320eb0fd1e2eed12c0b1f31ac9635c/execroot/__main__/bazel-out/darwin-fastbuild/testlogs/go_default_test/test.log

INFO: Build completed, 1 test FAILED, 2 total actions

BUILD.bazel

load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")

go_library(
    name = "go_default_library",
    srcs = ["main.go"],
    importpath = "github.com/example/project",
    visibility = ["//visibility:private"],
    x_defs = {
      "github.com/example/project.simple": "{BUILD_USER}",
      "github.com/example/project.compound": "{BUILD_USER}@{BUILD_HOST}",
    },
)

go_binary(
    name = "project",
    embed = [":go_default_library"],
    visibility = ["//visibility:public"],
)

go_test(
    name = "go_default_test",
    srcs = ["main_test.go"],
    embed = [":go_default_library"],
)

main.go

package main

var simple = "unknown"
var compound = "unknown"

func main() {}

main_test.go

package main

import "testing"

func TestStamp(t *testing.T) {
	if compound == "unknown" || simple == "unknown" {
		t.Errorf("compound = %q; simple = %q", compound, simple)
	}
}

WORKSPACE

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "io_bazel_rules_go",
    url = "https://github.com/bazelbuild/rules_go/releases/download/0.18.5/rules_go-0.18.5.tar.gz",
    sha256 = "a82a352bffae6bee4e95f68a8d80a70e87f42c4741e6a448bec11998fcc82329",
)
load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
go_rules_dependencies()
go_register_toolchains()