batches: support volume mode on Apple Silicon

Created by: courier-new

Tested basic "hello world" batch spec with volume mount mode (both setting the workspace flag and on auto with the new default condition) and didn't see any issues with the execution or the diffs produced. The new condition will default to volume mode for Intel and M1 Macs and keep bind mode as the default for anything else. I updated some comments in the area to capture the discussion @LawnGnome and I had about eventually moving to volume mode as the default for everyone.

Closes https://github.com/sourcegraph/sourcegraph/issues/27549