arcem-riscos.zip build artifact doesn't contain RISC OS filetype metadata

[Phlamethrower]

It looks like the build-riscos workflow job is relying on the github upload-artifact action to produce the zip file, resulting in a zip which contains unix-style filenames and no RISC OS filetype metadata.

Presumably there's a RISC OS-aware copy of zip included in the build docker image which we can use to create the zip archive instead of relying on github to do it for us.