Include `arm64` alias for `aarch64`

[mofeing]

When I use act to try some workflows, it fails to recognise the architecture in my Apple M1.

[Register Package/register]   ⭐ Run Main julia-actions/setup-julia@v1
[Register Package/register]   🐳  docker cp src=/Users/mofeing/.cache/act/julia-actions-setup-julia@v1/ dst=/var/run/act/actions/julia-actions-setup-julia@v1/
[Register Package/register]   🐳  docker exec cmd=[node /var/run/act/actions/julia-actions-setup-julia@v1/dist/index.js] user= workdir=
[Register Package/register]   💬  ::debug::platform: linux
[Register Package/register]   💬  ::debug::Downloading https://julialang-s3.julialang.org/bin/versions.json
[Register Package/register]   💬  ::debug::Downloading /tmp/d5ad4feb-e028-42b0-8167-086568e429c6
[Register Package/register]   💬  ::debug::download complete
[Register Package/register]   💬  ::debug::selected Julia version: undefined/1.6.7
[Register Package/register]   💬  ::debug::isExplicit: 1.6.7
[Register Package/register]   💬  ::debug::explicit? true
[Register Package/register]   💬  ::debug::checking cache: /opt/hostedtoolcache/julia/1.6.7/arm64
[Register Package/register]   💬  ::debug::not found
[Register Package/register]   💬  ::debug::could not find Julia undefined/1.6.7 in cache
[Register Package/register]   ❗  ::error::
[Register Package/register]   ❌  Failure - Main julia-actions/setup-julia@v1
[Register Package/register]   ⚙  ::set-output:: julia-version=1.6.7
[Register Package/register] exitcode '1': failure
[Register Package/register] 🏁  Job failed

Problem is that ${{ runner.arch }} in Apple M-series is defined as arm64 but only ARM64 and aarch64 are defined.

This PR should fix arch lookup in Apple M-series.

[SaschaMann]

Thanks!