ant: add an `instruction` element so we can pass context sensitive bnd instructions to `aQute.bnd.ant.BaseTask`

[rotty3000]

... so that we don't have to use properties which have immutability and visibility implications for the entire ant project.

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[github-actions[bot]]

This issue has been automatically closed due to inactivity. If you can reproduce this on a recent version of Bnd/Bndtools or if you have a good use case for this feature, please feel free to reopen the issue with steps to reproduce, a quick explanation of your use case or a high-quality pull request.