add runner.arch cache key in bats-action

[akhilerm]

Currently the bats-action caches the binary with the key {{ runner.os }}-bats-${{ inputs.bats-version }}. This can cause issues with cacheing in CI were multiplatform setup is used. Ref: https://github.com/opencontainers/runc/actions/runs/11013141253/job/30588072588?pr=4405#step:17:23.

The cache key should also have an entry for {{ runner.arch }} along with OS.

[brokenpip3]

2.1.0 just released, thanks @akhilerm