cmd/age: fix testscript setup races

cmd/age/age_test.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sync"
 	"testing"
 
 	"filippo.io/age"
@@ -50,12 +51,9 @@ func (testPlugin) Unwrap(ss []*age.Stanza) ([]byte, error) {
 	return nil, age.ErrIncorrectIdentity
 }
 
-func TestScript(t *testing.T) {
-	testscript.Run(t, testscript.Params{
-		Dir: "testdata",
-		Setup: func(e *testscript.Env) error {
+var buildExtraCommands = sync.OnceValue(func() error {
 	bindir := filepath.SplitList(os.Getenv("PATH"))[0]
-			// Build age-keygen and age-plugin-pq into the test binary directory
+	// Build age-keygen and age-plugin-pq into the test binary directory.
 	cmd := exec.Command("go", "build", "-o", bindir)
 	if testing.CoverMode() != "" {
 		cmd.Args = append(cmd.Args, "-cover")
@@ -65,6 +63,13 @@ func TestScript(t *testing.T) {
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	return cmd.Run()
+})
+
+func TestScript(t *testing.T) {
+	testscript.Run(t, testscript.Params{
+		Dir: "testdata",
+		Setup: func(e *testscript.Env) error {
+			return buildExtraCommands()
 		},
 		// TODO: enable AGEDEBUG=plugin without breaking stderr checks.
 	})