ci: support downloading micro_ros_motoplus prereleases

[gavanderhoorn]

As per title really.

Instead of just assuming / hard-coding release as the tag/release prefix, this allows adding a key called prerelease to the maps in uros. If that key is found, the rest of the workflow will assume it'll be downloading a prerelease micro_ros_motoplus archive.

This could be from an actually published prerelease, or a draft.

Example (and fictional) matrix entry:

uros: [
  { release: 20221102, codename: foxy, prerelease: true },
  ...,
]