ci: set commit-lint step as optional

[victorgz]

Description

Set the commit-lint CI step as optional.
As we're squashing the commits in all the Pull Requests and using the PR title as the new one, it is not actually relevent if the names of the PR commits are following the angular convention or not.

Motivation and Context

People contributing might not follow the Angular convention for the commit names, but at the end, what is important for us is the PR title (because of Squash & Merge). Examples:
#46
#47

How Has This Been Tested?

Test Case: the PR includes a commit not compliant with the Angular convention. Commit name: "Test optional step with non compliant commit"
Result: the step outcome is marked as success, while the error is displayed in the step details. More info about step context here

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[scolladon]

LGTM