Release v8

[thespad]

Lots of updates:

• Allow disabling image signing
• Support multiple branches ("build" workflow needs to be added to the target branch, baseimage & release check workflows need to stay in the default branch due to how github processes actions)
• Add riscv64 support
• Fix branch tags
• Write release version to repo file to avoid actions being disabled for lack of repo activity

Required changes to consuming repos:

• Update branch input to branch_name in baseimage & release check workflows
• Add branch_name inputs to build workflows if needed