Adjust how signing task is enabled/disabled

[marko-bekhta]

This should be closer to what's happening in the 6.6 branch.
Signing tasks are present when you try to publish to maven local, even though not requested explicitly. I initially thought about ignoring signing when there's a PublishToMavenLocal task present, but I realised that 6.6 uses a similar script, and that's the approach taken there.

as for the okhttp-digest turned out, it's already backported to 6.2, so nothing to be done here.

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[marko-bekhta]

closing as there's a better fix to be applied 😉