Don't pass synthesized TMPDIR if not already set (#409)

## Changes

On Unix systems, the default of `/tmp` always works. No need to
synthesize a path for it.

The custom TMPDIR was causing issues when used from GitHub Actions
runners.

## Tests

Confirmed manually this fixes the issue on GitHub Actions runners.

bundle/deploy/terraform/init.go

@@ -95,14 +95,9 @@ func setTempDirEnvVars(env map[string]string, b *bundle.Bundle) error {
 			env["TMP"] = tmpDir
 		}
 	default:
+		// If TMPDIR is not set, we let the process fall back to its default value.
 		if v, ok := os.LookupEnv("TMPDIR"); ok {
 			env["TMPDIR"] = v
-		} else {
-			tmpDir, err := b.CacheDir("tmp")
-			if err != nil {
-				return err
-			}
-			env["TMPDIR"] = tmpDir
 		}
 	}
 	return nil

bundle/deploy/terraform/init_test.go

@@ -69,7 +69,7 @@ func TestSetTempDirEnvVarsForUnixWithTmpDirSet(t *testing.T) {
 	err := setTempDirEnvVars(env, b)
 	require.NoError(t, err)
 
-	// assert that we pass through env var value
+	// Assert that we pass through TMPDIR.
 	assert.Equal(t, map[string]string{
 		"TMPDIR": "/foo/bar",
 	}, env)
@@ -97,12 +97,8 @@ func TestSetTempDirEnvVarsForUnixWithTmpDirNotSet(t *testing.T) {
 	err := setTempDirEnvVars(env, b)
 	require.NoError(t, err)
 
-	// assert tmp dir is set to b.CacheDir("tmp")
+	// Assert that we don't pass through TMPDIR.
-	tmpDir, err := b.CacheDir("tmp")
+	assert.Equal(t, map[string]string{}, env)
-	require.NoError(t, err)
-	assert.Equal(t, map[string]string{
-		"TMPDIR": tmpDir,
-	}, env)
 }
 
 func TestSetTempDirEnvVarsForWindowWithAllTmpDirEnvVarsSet(t *testing.T) {