fix update_archlinux_keyring input

[ReenigneArcher]

Fixes the input variable name of the update_archlinux_keyring input, in the entrypoint.sh. Additionally, fixes the command used to update the keyring.

While investigating #32, I discovered that the this section of the script was not running.

I have done my best to test this by replicating your repo (here: https://github.com/LizardByte/archlinux-package-action), including the image published to ghcr. It's a bit difficult to test another branch. If possible, I'd suggest that the action wouldn't rely on a published docker image. Maybe it could just build the image when the action runs? Anyway, I've tested the image in my workflow, here are the results: https://github.com/LizardByte/Sunshine/actions/runs/3606596462/jobs/6078056553#step:7:61

I believe this fixes #32

[ReenigneArcher]

FYI, I am planning to move away from this action as I want to reduce maintenance burden on my GitHub org and don't plan to maintain or keep my fork of this repo. I will likely be deleting my fork in the next week or two. I can keep it a little while longer if you acknowledge my 3 PRs. Otherwise once I delete the repo it will close the PRs and you will not be able to merge them.

P.S. I also have other improvements in my branch, such as the action will use the dockerfile in the branch you specify when running the action instead of some dockerfile in another location that is not related to the version of the action. It's really pretty quick to build the image ( < 1 minute ) so I don't see any drawback to this method.