Improved manual build flow to make it easier

[NomDeTom]

This PR condenses the process of building a single firmware in github actions down to selecting the branch and inserting the variant. Additional checks are inserted to ensure that a valid target is chosen, and further guidance on selecting a valid target.

🤝 Attestations

• [x] I have tested that my proposed changes behave as described.
• [ ] I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • [ ] Heltec (Lora32) V3
  • [ ] LilyGo T-Deck
  • [ ] LilyGo T-Beam
  • [ ] RAK WisBlock 4631
  • [ ] Seeed Studio T-1000E tracker card
  • [ ] Other (please specify below)

[NomDeTom]

@vidplace7 have a go with this one. Would it be a candidate to pull through to Main?