Fix syntax errors in Kissat checking script

[tautschnig]

Description of changes:

In bash, return can only be used within a function. Use exit instead.

Resolved issues:

n/a

Related RFC:

n/a

Call-outs:

n/a

Testing:

• How is this change tested? Locally

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• n/a Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[jaisnan]

Thanks for catching this

[zhassan-aws]

Thanks