build: use git describe fallback
build: use git describe fallback

Pass `--always` to `git describe`, falling back to commit revision
in case tags were not fetched.
This workarounds some non-deterministic race/failure on semaphore:
```
fatal: No names found, cannot describe anything.
```

OMWAVE/docker2aci,appc/docker2aci,OMWAVE/docker2aci,dgonyeo/docker2aci,appc/docker2aci,dgonyeo/docker2aci