Switch to podman-static for podman installation

[bduffany]

Switch from alvistack to a fork of https://github.com/mgoltzsche/podman-static (the fork lives at https://github.com/buildbuddy-io/podman-static). Also bump the podman version to v4.9.4, since this repo appears to be missing 4.9.1 (but otherwise has a good track record of releasing new versions frequently).

This has multiple benefits:

• Includes linux amd64 as well as arm64 builds (alvistack does not)
• Is open source (I could not find the source code for the alvistack debian packaging)
• Has a stable version history so far (alvistack only retains a small number of the most recent podman versions)
• Is portable and has no external deps (due to being statically linked) and has easy/fast installation, which means we can improve how we run our podman integration tests. Specifically we can have podman_test extract the tar under / before running the test, so that we're always testing the podman version at the current git commit, rather than depending on an outdated executor image that doesn't reflect the current podman version. See the conversation here. (We could in theory do something similar with alvistack, but it would be significantly slower and more complicated.)
• Results in a slightly smaller executor image size (reduces the current image size from 1.19GB => 1.09GB)

This repo seems to be well-maintained. It is missing podman v5+ builds but v5 has only been out for a couple of weeks.

Tested in dev with some basic actions, and we'll also have this week to do more testing.

Related issues: N/A