Set limit for execution in C# linter

Summary: This sets the limit for future execution in the C# linter to 8.  See D7606 for more information.

Test Plan:
Ran

```
arc lint --everything --trace --never-apply-patches --output json
```

and saw it only run 8 commands at once.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7607