ember-cli-update creates new app in /tmp

Running ember-cli-update within an existing app creates a new app inside /tmp instead. Here is the output:
```
installing app
Ember CLI v3.17.0

Creating a new Ember app in /tmp/tmp-7917c6mBjlmacDJS/ui:
installing app
Ember CLI v3.17.0

Creating a new Ember app in /tmp/tmp-7917ZQ0Uw6FQX72J/ui:
create .editorconfig
create .ember-cli
create .eslintignore
create .eslintrc.js
create .template-lintrc.js
create .travis.yml
create .watchmanconfig
create README.md
create app/app.js
create app/components/.gitkeep
create app/controllers/.gitkeep
create app/helpers/.gitkeep
create app/index.html
create app/models/.gitkeep
create app/router.js
create app/routes/.gitkeep
create app/styles/app.css
create app/templates/application.hbs
create config/environment.js
create config/optional-features.json
create config/targets.js
create ember-cli-build.js
create .gitignore
create package.json
create public/robots.txt
create testem.js
create tests/helpers/.gitkeep
create tests/index.html
create tests/integration/.gitkeep
create tests/test-helper.js
create tests/unit/.gitkeep
create vendor/.gitkeep

Successfully created project ui.
Get started by typing:

$ cd ui
$ npm start

Happy coding!

```
The original package.json looks like so:
```
{
"name": "swd-ui",
"version": "0.0.0",
"private": true,
"description": "Small description for swd-ui goes here",
"repository": "",
"license": "MIT",
"author": "",
"directories": {
"doc": "doc",
"test": "tests"
},
"scripts": {
"build": "ember build",
"lint:hbs": "ember-template-lint .",
"lint:js": "eslint .",
"start": "ember serve",
"test": "ember test"
},
"devDependencies": {
"@ember/optional-features": "^1.0.0",
"babel-eslint": "^10.0.3",
"broccoli-asset-rev": "^3.0.0",
"ember-cli": "~3.13.1",
"ember-cli-app-version": "^3.2.0",
"ember-cli-babel": "^7.11.1",
"ember-cli-dependency-checker": "^3.1.0",
"ember-cli-eslint": "^5.1.0",
"ember-cli-htmlbars": "^4.0.0",
"ember-cli-htmlbars-inline-precompile": "^3.0.0",
"ember-cli-inject-live-reload": "^2.0.1",
"ember-cli-sri": "^2.1.1",
"ember-cli-template-lint": "^1.0.0-beta.3",
"ember-cli-uglify": "^3.0.0",
"ember-data": "~3.13.0",
"ember-export-application-global": "^2.0.0",
"ember-fetch": "^6.7.0",
"ember-load-initializers": "^2.1.0",
"ember-maybe-import-regenerator": "^0.1.6",
"ember-qunit": "^4.5.1",
"ember-resolver": "^5.3.0",
"ember-source": "~3.13.0",
"ember-welcome-page": "^4.0.0",
"eslint-plugin-ember": "^7.1.0",
"eslint-plugin-node": "^10.0.0",
"loader.js": "^4.7.0",
"qunit-dom": "^0.9.0"
},
"engines": {
"node": "8.* || >= 10.*"
}
}
```
Versions of libraries:
npm 6.13.4
ember-cli: 3.17.0
node: 12.16.1

使用报告中的 package.json、npm 6.13.4、Node 12.16.1 以及 ember-cli-update 的 Ember CLI 3.17.0 环境重现该问题。从 CLI 入口点跟踪更新命令，并将其在现有 app 中的行为与报告的嵌套 /tmp app 创建行为进行比较；当现有 app 被更新且不会在那里创建新 app 时，即表示完成。

由索引模型根据 Issue 内容生成。