add nightly github action

[cwisniew]

Identify the Bug or Feature request

Resolves #4450

Description of the Change

Creates nightly builds of MapTool

Possible Drawbacks

• The cron based actions for GitHub are known to be flakey, they will not run if the GitHub actions platform is too busy, I have tried to mitigate this by running it at an odd time (i.e. not an hour or half hour or 20 or 15 minute multiple when many other cron based jobs might be running). We will see how it goes, but its not the end of the world if the odd build doesn't get created while we are trialling it. (It was actually a bit fiddly to get working until google pointed me to comments saying it wont run at busy periods)
• This script does not delete old nightlies but for now, I can manage that manually by deleting them every now and again.
• Nightlys will get build even if there are no changes.

Documentation Notes

Nightly MapTool Builds are now available

Release Notes

• Nightly MapTool Builds are now available

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This change is