ci: Fix tags on ad hoc unitctl releases

[callahad]

• Adds unitctl/ prefix to tags generated by manual workflow runs. Previously, only release titles (but not tags) were prefixed.

• Omits superfluous name field; falls back to tag when absent.

• Removes unnecessary conditional from prelease field.

This results in the following tagging / releasing behavior:

1. Running manually creates a pre-release and tags it unitctl/VERSION
2. Pushing a tag formatted like x.y.z creates a normal release

Refines: 3501a50ffb93756e145295021ff9313ac77f1ba9

[callahad]

One other important part of this: Since previously, all releases were unconditionally titled with the "unitctl/" prefix, pushing a release tag (e.g., 1.33.0) would create a GitHub Release titled "unitctl/1.33.0".

Feels like when we make a release of Unit, the GitHub Release should be titled "1.33.0" without the "unitctl/" prefix.