RPM and PKG packages need license information

### Describe the bug

RPM and PKG packages include a short descriptor for the license, but after #1812 Briefcase will no longer know that (it either has a licence file or a licence text which may or may not be a short descriptor). Consequently, it is always set to "Unknown".

Prior to #1812 briefcase *might* have a way to know the license descriptor, but it is no guarantee that it matches the LICENSE file.

### Steps to reproduce

1. Build an RPM or PKG bundle
2. Look at the spec file
3. Observe that the "License" field is set to "Unknown"

### Expected behavior

A short license descriptor is included in the RPM spec file and the PKGBUILD-file

### Screenshots

_No response_

### Environment

Any environment

### Logs

_No response_

### Additional context

_No response_

Start by locating the code that generates RPM spec files and PKGBUILD files, then trace how the license value is obtained after #1812. Confirm the generated RPM spec and PKGBUILD contain a short license descriptor rather than "Unknown", and run the relevant packaging tests if present.

Written by the indexing model from the issue text.