Bug: `matrix` valid as job-parameter

- [x] I have read [Contribution Guidelines](https://github.com/CircleCI-Public/circleci-cli/blob/master/CONTRIBUTING.md).
- [x] I have checked for similar issues and haven't found anything relevant. ([Repo-wide search for `matrix` returned no results](https://github.com/CircleCI-Public/circleci-cli/search?q=matrix&unscoped_q=matrix))
- [x] This is not a security issue (which should be reported here: https://circleci.com/security/)

**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**

You can set a job-parameter named `matrix` and `circleci config validate` succeeds ("Config file at .circleci/config.yml is valid.").

MWE:
```yaml
workflows:
test-workflow:
jobs:
- test-orb/test-job

orbs:
test-orb:
jobs:
test-job:
parameters:
matrix: # <---- this shouldn't be possible
type: string
default: hello
docker:
- image: alpine
steps:
- run: echo hello world

version: 2.1

```
**What is the expected behavior?**

Given that `matrix` is now a reserved word in version 2.1, the validation should return with an error.

**Which version of the CLI and OS are you using? Did this work in previous versions?**

`0.1.7868+fa5217e (release)`

```
$ circleci diagnostic

---
CircleCI CLI Diagnostics
---
Debugger mode: false
Config found: /Users/dominik/.circleci/cli.yml
API host: https://circleci.com
API endpoint: graphql-unstable
OK, got a token.
Trying an introspection query on API...
Ok.
```

**If you have any questions, feel free to ping us at @CircleCI-Public/x-team.**