add init-pants action `input.experimental-remote-cache-via-gha` (#30)

This makes the documented workflow step available via a simple boolean
input flag:

https://www.pantsbuild.org/2.20/docs/using-pants/remote-caching-and-execution/remote-caching#workflow

Also, I noticed the README was out of date, so I updated it.

ci: Bump setup-python from v4 to v5 (#29)

https://github.com/actions/setup-python/releases/tag/v5.0.0
setup-python@v4 is using the deprecated node16 version.
Upgrade to setup-python@v5 version using node20 version.

update GH cache action (#28)

- v3 still uses node 16 which is deprecated
- see
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/
- see https://github.com/actions/cache/releases/tag/v4.0.0

init-pants: Support new `get-pants.sh` that installs in `~/.local/bin…

…` instead of `~/bin` (#26)

The newest version of `get-pants.sh` uses `~/.local/bin`:
pantsbuild/setup#144

This PR has backwards-compatible support for older in-repo copies of the
script that still use `~/bin`, but it defaults to the newer
`~/.local/bin`.

Don't download `get-pants.sh` into repo root. (#18)

Downloading into the repo root will cause subsequent `pants tailor --check`
calls to fail (unless there happens to be a `shell_sources` target in the build root).
Download into `/tmp` instead.

Ensure get-pants.sh is runnable after downloading it. (#17)

Update the init-pants action to work with scie-pants. (#16)

Fix cache key/restore key mismatch. (#14)

Upgrade actions to fix `set-output` warnings (#10)