add post-config script

[zshannon]

Adding support for a post-config script that runs during GH runner setup.

Motivation and Context

I need to modify the github runner's binary to modify its actions/cache behavior like this and figured a post-config script could be generally useful.

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)

[zshannon]

I ended up needing to hardcode the ACTIONS_CACHE_URL value in a custom build of the actions runner so perhaps this PR isn't helpful, feel free to close.

If you wanna explore building caching in here, happy to share my experience eventually getting it to work.

[simonbs]

@zshannon Thanks for opening the PR! I understand that you needed to hardcode ACTIONS_CACHE_URL but did you also end up relying on the post-config script or are you no longer using it?

[zshannon]

I ended up using the cirruslabs/cache action which redefines ACTION_CACHE_URL at cache execution time (but with my own CIRRUS_HTTP_CACHE_HOST value pointing at an instance of gha-cache-server on my network)

[simonbs]

Alright. I think the changes in this PR are a great fit for Tartelet but I’d prefer not introducing it until anyone’s actually using it, so we don’t have to test and maintain unused features.

Therefore, I’ll close the PR but certainly do feel free to reopen it if you ever need this in Tartelet 😊