Fix building VAST within a shallow git tree

[dominiklohmann]

📔 Description

This change makes it so we warn instead of fail on git-describe errors, falling back to the specified fallback version. This helps in situation where git and the source tree are available, but git-describe fails, e.g., when working from within a shallow clone.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

n/t

[lava]

Nice!