Use a proper fallback when git describe fails

[wilzbach]

Due to #6940 the fix from #6941 is also needed for master, see e.g. dlang/phobos#5514

I cherry-picked the same commit to keep things simple as I would like to get CircleCi working at Phobos.

CC @MartinNowak @CyberShadow

Using VERSION without anything else seems a bit more misleading as it can then be confused with a tagged release.

Usually git describe shouldn't fail and if it's probably a source ball archive. Anyhow my main focus here is really fixing CircleCi - we can always change the behavior in a follow-up.

[dlang-bot]

Thanks for your pull request, @wilzbach! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

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• smaller, focused PRs are easier to review than big ones

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[MartinNowak]

#6941 already went into master, I picked 89beb11 over to stable

[wilzbach]

I picked 89beb11 over to stable

Thanks, Martin!