Action gives 404 when downloading opa for arm64

[julioz]

Starting on v0.37.0 of opa, arm64 binary has a static suffix and so this action cannot find the binary to be downloaded when following the repository's README.

$ act
[conftest/conftest] 🚀  Start image=catthehacker/ubuntu:act-latest
[conftest/conftest]   🐳  docker pull image=catthehacker/ubuntu:act-latest platform= username= forcePull=true
...
[conftest/conftest] ⭐ Run Main Setup OPA
[conftest/conftest]   🐳  docker cp src=/Users/julio/.cache/act/open-policy-agent-setup-opa@v2/ dst=/var/run/act/actions/open-policy-agent-setup-opa@v2/
[conftest/conftest]   🐳  docker exec cmd=[node /var/run/act/actions/open-policy-agent-setup-opa@v2/dist/index.js] user= workdir=
[conftest/conftest]   💬  ::debug::Downloading https://github.com/open-policy-agent/opa/releases/download/v0.54.0/opa_linux_arm64
[conftest/conftest]   💬  ::debug::Destination /tmp/tmpILgatL/opa_linux_arm64
[conftest/conftest]   💬  ::debug::Failed to download from "https://github.com/open-policy-agent/opa/releases/download/v0.54.0/opa_linux_arm64". Code(404) Message(Not Found)
[conftest/conftest]   ❗  ::error::Error: Unexpected HTTP response: 404
[conftest/conftest]   ❌  Failure - Main Setup OPA
[conftest/conftest] exitcode '1': failure
[conftest/conftest] 🏁  Job failed

We should update the documentation to reflect the code changes.

[anderseknert]

+1 for adding an example of using the static attribute to the README / docs. Would you want to submit a PR?
Sorry for the very long delay btw, I hadn't seen this before!