Add pixi-lock/create-and-cache and pixi-lock/restore actions.

[VeckoTheGecko]

Take 2, followup from #1 .

See changes (including new README) for full details.

Now that its split into two actions with workflow dependencies , we can avoid race conditions.

Action structure:

• pixi-lock/create-and-cache (run in a separate job - e..g, cache-pixi-lock)
  • takes pixi-version as input
  • checkout
  • creates a cache key of pixi-lock_${{ inputs.pixi-version }}_${{ hashFiles('pixi.toml') }}_${today}
  • checks if there's an existing cache with this key
  • if yes: skip the rest of the steps
  • sets up pixi
  • runs pixi lock
  • saves the lock file under the cache key
  • output:
    • pixi-version and cache-key (the output from this needs to be propagated up to the job level)
• other jobs
  • pixi-lock/restore restores the pixi.lock file
    • takes in the cache-key from the cache-pixi-lock job
    • calculates a fallback key by calculating the key for the previous day (avoiding edge case of midnight runs)
    • restore the pixi.lock file
  • setup-pixi
    • run with the pixi-version that was output from the cache-pixi-lock job

TODO:

• Rename repo to pixi-lock instead of create-pixi-lock

[VeckoTheGecko]

Merging for now so that we can test it in action in the Parcels repo. @keewis let me know if there's any updates we can make. I was talking to @wolfv and they're happy to take ownership of it under prefix-dev