feat: Allow to retry pixi install

[borchero]

In some instances, running this action on self-hosted runners in an environment with bad network connection requires retries of pixi install regardless of pixi's internal retries. This PR introduces a separate flag retry-count which allows to retry this step with exponential backoff.

If updating documentation:

• Updated documentation in https://github.com/prefix-dev/pixi/blob/main/docs/advanced/github_actions.md as well

[pavelzw]

As discussed in private, I don't think a lot of users will benefit from this workaround and thus we implemented this in an internal fork.
If more people experience this issue, feel free to comment here and we could reconsider opening this (or ideally getting behind the root cause).

xref prefix-dev/pixi#2336, prefix-dev/pixi#1994