Add macOS support

[ankushg]

I tried this orb in a macOS executor and it hangs while trying to cat $CHECKOUT_KEY

When I connected via SSH, I could reproduce the hang when manually running cat $CHECKOUT_KEY, but things were fine when running echo instead

I'm not sure how to validate that this fixes everything though 😅

In this PR

• switch from cat to echo for $CHECKOUT_KEY
• add macOS integration tests to config.yml to make sure things work (and keep working)
• add macOS to @orb.yml (unchecked for now haha)

[ankushg]

@guitarrapc I'm not sure if there's any way for external contributors to run integration tests, or cut an alpha build, but would love to see if this works for my macOS instance 😄

We were previously using https://github.com/vsco/advanced-checkout-orb (which worked for us on macOS!) but your fetch_options is exactly what we need

[guitarrapc]

@ankushg Thank you for your contributions.

I'm not sure if there's any way for external contributors to run integration tests, or cut an alpha build, but would love to see if this works for my macOS instance

I also don't know how to release alpha with external contribution 🤣 However code seems correct and perhaps work on both linux and macOS.

Let me merge you PR and I will do test.

[guitarrapc]

OK, it seems like I need get support with CircleCI to run orb test on macOS #19.

However Linux seems ok and vsco/advanced-checkout-orb also uses echo instead of cat, let's publish orb.

[guitarrapc]

@ankushg I've released orb v2.3.0. Please try it.

https://circleci.com/developer/orbs/orb/guitarrapc/git-shallow-clone?version=2.3.0

note: As of pipeline issue, I've disabled macOS integration test until CirrcleCI gives OSS offer, will enable it right after then.

[guitarrapc]

Re-enabled macOS test on #22, thanks.