Add some kind of environment marker indicating success?

[woodruffw]

At some point (probably soon), uv will itself be capable of generating attestations. At that point, this action will become obsolete. However, there will probably be a tail of people using it, and they'll potentially be confused when the combination of astral-sh/attest-action + uv publish causes warnings (or errors) about conflicting attestations.

To ameliorate this, we could probably have this action set an environment variable, something like ASTRAL_SH_ATTEST_ACTION_MANAGED=1, which uv publish could then check for and skip attestation generation when present.

Not sure if this is a good idea.