release: can't compile if `describe` is empty

[BradLugo]

The fix provided by #1833 doesn't appear to work due to describe being a const.

Steps to reproduce

In cmd/build.go, set describe to an empty string. Then try to build:

❯ go build \
    -trimpath -ldflags="-s -w" -buildvcs=false \
    -o "clairctl-windows-arm64" \
    ./cmd/clairctl
# github.com/quay/clair/v4/cmd
cmd/build.go:88:37: invalid argument: index 0 out of bounds [0:0]

[BradLugo]

fwiw, the reason I ran into this issue was due to attempting to run the Release workflow by pushing a tag to my fork (to provide a community member a way to easily build and download a binary), so I'm slightly concerned we might run into this issue during the next release. Here's the workflow run I mentioned: https://github.com/BradLugo/clair/actions/runs/15782791640

[hdonnay]

If you change the contents of the string the code processing the string will break, yeah.

[BradLugo]

This occurred from pushing a lightweight tag to a fork (i.e., no code changes on my part). The steps to reproduce describe what happens in CI, but are manually done.

[hdonnay]

Does adding ,tags to the describe options make a lightweight tag work?

We haven't run into the lightweight tag issue (beyond GH Actions fucking with the tag) because we just never used them.