Architect#scheduleBuilder or context.scheduleBuilder results in incorrect target on context

# 🐞 Bug report

### Command (mark with an `x`)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] extract-i18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Is this a regression?

Not that I know of

### Description

If a builder is executed via `architect.scheduleBuilder`, that builder is passed a context that doesn't match the declared interface in typescript. The same happens when using `context.scheduleBuilder` if no target is passed via the `scheduleOptions`.

```js
console.log(context.target);
// logs {project: undefined, target: undefined, configuration: undefined}
// but type is {project: string, target: string, configuration?: string} | undefined
```

## 🔬 Minimal Reproduction

Run `yarn bazel test //packages/angular_devkit/architect:architect_test` with the following patch:

```patch
diff --git a/packages/angular_devkit/architect/src/index_spec.ts b/packages/angular_devkit/architect/src/index_spec.ts
index 57d76c5a6..bbd59b3fc 100644
--- a/packages/angular_devkit/architect/src/index_spec.ts
+++ b/packages/angular_devkit/architect/src/index_spec.ts
@@ -36,10 +36,12 @@ describe('architect', () => {

options = {};
called = 0;
- testArchitectHost.addBuilder('package:test', createBuilder(async o => {
+ testArchitectHost.addBuilder('package:test', createBuilder(async (o, ctx) => {
called++;
options = o;

+ expect(ctx.target != null && ctx.target.target == null).toBe(false);
+
return new Promise(resolve => {
setTimeout(() => resolve({ success: true }), 10);
});
```

## 🔥 Exception or Error

Failures:

1) architect runs builders parallel

  Message:

    Expected true to be false.

  Stack:

    Error: Expected true to be false.

        at 

        at packages/angular_devkit/architect/src/index_spec.ts:43:63

        at onInput (packages/angular_devkit/architect/src/create-builder.ts:201:20)

        at SafeSubscriber._next (packages/angular_devkit/architect/src/create-builder.ts:87:17)

## 🌍 Your Environment

Master branch on this repo

**Anything else relevant?**

Beginne mit packages/angular_devkit/architect/src/index_spec.ts und reproduziere den Fehler mit yarn bazel test //packages/angular_devkit/architect:architect_test. Verfolge den Builder-Kontext durch packages/angular_devkit/architect/src/create-builder.ts, auf die der Stack verweist, und überprüfe, dass scheduleBuilder ein Ziel bereitstellt, das seiner deklarierten TypeScript-Schnittstelle entspricht. Erledigt ist es, wenn der fokussierte architect-Test mit einem gültigen Ziel besteht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.