build.d: check result of git describe

[wilzbach]

See also: #10232 (comment)

Though we should probably just build and call config.d as we have to maintain that one anyhow.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10244"

[wilzbach]

Requires dlang/ci#398 for buildkite as otherwise master is checked out for the Phobos as a project test.

[wilzbach]

Requires dlang/ci#398 for buildkite

Urgh and that one fails because DMD's version (with master) on Buildkite is now:

DMD64 D Compiler 5bfa3a2

(due to --depth 1 and --always)

So I'm going to have to force-merge this one.

[rainers]

Thanks @wilzbach