Dynamic version is removed from lockfile occasionally

[charliermarsh]

@charliermarsh Use this repo: https://github.com/apollo13/uv-bug

Follow this to the letter (I am not sure if you can repro if you do anything different):

• uv cache clean
• uv lock and observe that git status shows no changes
• uv sync
• rm -rf uv.lock .venv
• uv lock
• Run git diff and see the version written into the lock file (shouldn't be)
• uv cache clean
• rm -rf uv.lock .venv
• uv lock (No version written -> okay)

Originally posted by @apollo13 in #7533