Overrides do no work in PNPM monorepo

PNPM supports `pnpm.overrides` and `resolutions` only at the root of a project. Ember Try sets it at the closest `package.json`. In a typical setup of a v2 addon that's the "test-app" package. That does not have any affect. Ember Try must set it in the `package.json` at the workspace root.

Likely there is a second bug: Ember Try sets `overrides` at the root of the package.json. That's correct for NPM. But I think for PNPM it must go into a `pnpm` member. This is an example taken from PNPM documentation:

```json
{
"dependencies": {
"foo": "^1.0.0"
},
"pnpm": {
"overrides": {
"foo": "$foo"
}
}
}
```

But Ember Try generates the following, which is valid for NPM but does not fit with PNPM documentation:

```json
{
"dependencies": {
"foo": "^1.0.0"
},
"overrides": {
"foo": "$foo",
}
}
```