Clarify "ref" description in GitHub build workflow

[Sharpie]

This commit updates the description of the ref input in build.yml to indicate that pre-release builds can be produced by providing a branch or other git ref instead of a tag.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)