Per-tool option to check availability in environment

Some tools are only needed in certain environments or on certain platforms -- for example, something running on a GitHub runner should use the provided `gh` command, but this may not be available on a user's machine and needs to be downloaded. Or `grep` may not be available on Windows, but is on most other systems. If binny had the capability to check for the tool before downloading or executing, it would offer another way to improve performance and compatibility. This is important for use cases other than `binny install`.

Start by tracing the tool download and execution flows described in the issue, including use cases other than `binny install`. Determine how a per-tool availability check should be configured and applied across platforms. Done means tools can be checked before downloading or executing without breaking existing behavior.

Written by the indexing model from the issue text.