Use environment variable to set CODEOWNERS write path

[jibarra]

The current version of the gem always defaults CODEOWNERS to generate in .github/CODEOWNERS. This works well for projects that follow the same structure but doesn't work well for other structures (or other platforms). For example:

• Github works with CODEOWNERS in .github/, docs/ or root.
• Gitlab works with CODEOWNERS in .gitlab/, docs/ or root.

This change updates the path for the file location to pull from an environment variable (CODEOWNERS_PATH) to use for the path. It will still default to the old behavior of using .github/ to maintain backwards compatibility.

[ashleywillard]

This seems like a good change. Another way to implement this would be to specify the codeowners path in the code_ownership.yml file.

[jibarra]

@ashleywillard - I like that recommendation a little better. I'll see about doing that instead.

[jibarra]

@ashleywillard - How's this look now?

[ashleywillard]

Looks good! cc @perryqh