diff --git a/go/packages/packages.go b/go/packages/packages.go
index f40720d0..84a3dbb1 100644
--- a/go/packages/packages.go
+++ b/go/packages/packages.go
@@ -20,6 +20,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"sync"
 
 	"golang.org/x/tools/go/gcexportdata"
@@ -685,8 +686,11 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 	// This is only an approximation.
 	// TODO(adonovan): derive Sizes from the underlying build system.
 	goarch := runtime.GOARCH
-	if x, ok := os.LookupEnv("GOARCH"); ok {
-		goarch = x
+	const goarchPrefix = "GOARCH="
+	for _, e := range ld.Config.Env {
+		if strings.HasPrefix(e, goarchPrefix) {
+			goarch = e[len(goarchPrefix):]
+		}
 	}
 	sizes := types.SizesFor("gc", goarch)
 
diff --git a/go/packages/packages_test.go b/go/packages/packages_test.go
index 4da9bb8b..fedf6faf 100644
--- a/go/packages/packages_test.go
+++ b/go/packages/packages_test.go
@@ -1052,15 +1052,11 @@ func TestSizes(t *testing.T) {
 	})
 	defer cleanup()
 
-	savedGOARCH := os.Getenv("GOARCH")
-	defer os.Setenv("GOARCH", savedGOARCH)
-
 	for arch, wantWordSize := range map[string]int64{"386": 32, "amd64": 64} {
-		os.Setenv("GOARCH", arch)
 		cfg := &packages.Config{
 			Mode: packages.LoadSyntax,
 			Dir:  tmp,
-			Env:  append(os.Environ(), "GOPATH="+tmp, "GO111MODULE=off"),
+			Env:  append(os.Environ(), "GOARCH="+arch, "GOPATH="+tmp, "GO111MODULE=off"),
 		}
 		initial, err := packages.Load(cfg, "a")
 		if err != nil {