fix checkpoint disable propagation

fixes #2

tfexec/terraform.go

diff --git a/tfexec/terraform.go b/tfexec/terraform.go
index 9ee9501..97d745a 100644
--- a/tfexec/terraform.go
+++ b/tfexec/terraform.go
@@ -62,10 +62,9 @@
 	var tfenv []string
 
 	// always propagate CHECKPOINT_DISABLE env var unless it is
-	// explicitly overridden
-	c := os.Getenv("CHECKPOINT_DISABLE")
-	if c != "" {
-		tfenv = append(tfenv, "CHECKPOINT_DISABLE="+c)
+	// explicitly overridden with tf.SetEnv
+	if _, ok := env["CHECKPOINT_DISABLE"]; !ok {
+		env["CHECKPOINT_DISABLE"] = os.Getenv("CHECKPOINT_DISABLE")
 	}
 
 	for k, v := range env {

tfexec/terraform_test.go

diff --git a/tfexec/terraform_test.go b/tfexec/terraform_test.go
index b8216ff..dfe4419 100644
--- a/tfexec/terraform_test.go
+++ b/tfexec/terraform_test.go
@@ -19,6 +19,43 @@
 const testStateJsonFileName = "state.json"
 const testTerraformStateFileName = "terraform.tfstate"
 
+func TestCheckpointDisablePropagation(t *testing.T) {
+	td := testTempDir(t)
+	defer os.RemoveAll(td)
+
+	tf, err := NewTerraform(td, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// case 1: env var is set in environment and not overridden
+	os.Setenv("CHECKPOINT_DISABLE", "1")
+	defer os.Unsetenv("CHECKPOINT_DISABLE")
+	tf.SetEnv(map[string]string{
+		"FOOBAR": "1",
+	})
+	initCmd := tf.InitCmd(context.Background())
+	expected := []string{"FOOBAR=1", "CHECKPOINT_DISABLE=1", "TF_LOG="}
+	actual := initCmd.Env
+
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("expected command env to be %s, but it was %s", expected, actual)
+	}
+
+	// case 2: env var is set in environment and overridden with SetEnv
+	tf.SetEnv(map[string]string{
+		"CHECKPOINT_DISABLE": "",
+		"FOOBAR":             "1",
+	})
+	initCmd = tf.InitCmd(context.Background())
+	expected = []string{"CHECKPOINT_DISABLE=", "FOOBAR=1", "TF_LOG="}
+	actual = initCmd.Env
+
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("expected command env to be %s, but it was %s", expected, actual)
+	}
+}
+
 func TestInitCmd(t *testing.T) {
 	td := testTempDir(t)
 	defer os.RemoveAll(td)