Pull in the latest code from Go's template packages (#11771)

Fixes #10707
Fixes #11507

tpl/internal/go_templates/testenv/exec.go

@@ -60,13 +60,6 @@ func tryExec() error {
 	// may as well use the same path so that this branch can be tested without
 	// an ios environment.
 
-	/*if !testing.Testing() {
-		// This isn't a standard 'go test' binary, so we don't know how to
-		// self-exec in a way that should succeed without side effects.
-		// Just forget it.
-		return errors.New("can't probe for exec support with a non-test executable")
-	}*/
-
 	// We know that this is a test executable. We should be able to run it with a
 	// no-op flag to check for overall exec support.
 	exe, err := os.Executable()
@@ -99,11 +92,14 @@ func MustHaveExecPath(t testing.TB, path string) {
 // CleanCmdEnv will fill cmd.Env with the environment, excluding certain
 // variables that could modify the behavior of the Go tools such as
 // GODEBUG and GOTRACEBACK.
+//
+// If the caller wants to set cmd.Dir, set it before calling this function,
+// so PWD will be set correctly in the environment.
 func CleanCmdEnv(cmd *exec.Cmd) *exec.Cmd {
 	if cmd.Env != nil {
 		panic("environment already set")
 	}
-	for _, env := range os.Environ() {
+	for _, env := range cmd.Environ() {
 		// Exclude GODEBUG from the environment to prevent its output
 		// from breaking tests that are trying to parse other command output.
 		if strings.HasPrefix(env, "GODEBUG=") {