[sc-157321] Allow designating `dir` in github action

[jazanne]

This is completely optional, but is useful if someone checks out multiple repos in their action.

For example, this git-gatsby workflow used to check out multiple repos internal repos

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Dev notes

this must use env override and not github action input because of how env variable is used to find the configuration file etc.

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #157321: github action should allow dir to be specified.

[jazanne]

@mmrj i had to change how this works, sorry to make ya re-review!

[jazanne]

@InTheCloudDan wasn't sure how to test this since, if you have thoughts i'd love to hear them.

[InTheCloudDan]

@jazanne yeah this is an interesting one, I'm not really sure what would be the best way to it. Maybe just a single unit test confirming what's set is correctly passed in??

[jazanne]

not high priority