fix: respect pre-existing .env files when preparing the herokuish buildenv

Closes #4894

tests/unit/builder-herokuish.bats

@@ -0,0 +1,19 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  create_app
+}
+
+teardown() {
+  destroy_app
+}
+
+@test "(builder-herouish:build .env)" {
+  run deploy_app python dokku@dokku.me:$TEST_APP
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains 'DOTENV_KEY=some_value'
+}