The when statement is supposed to be on the block, not on the individual task.

It affects all tasks within the block (they're all to be executed when ma1sd is enabled and self-building is requested0.

The tag format used in the `ma1sd` repo have change. Versions no longer
start with 'v', and when building for non-amd64, we also need to strip
off the '-$arch' bit from the Docker image name.

Further, when building the .jar file, `ma1sd` currently names the .jar
based on the project's directory, which we call 'docker-src'. This means
other parts of the `ma1sd` build can't find the .jar file. Remedy this
by ensuring that the dir is called `docker-src/ma1sd`.

No migration steps should be required.

(adapting strings and URLs)