Packages that consume pdm-backend often use it, quite similary to
setuptools-scm-version, for dynamic versioning. That works fine when
pulling releases from PyPi, but needs special handling, when releases
are pulled from Git.
The pdm-backend package will now always export `PDM_BUILD_SCM_VERSION=`
to propagate the derivation version to pdm-backend.
This behaviour can be disabled by setting
```
dontSetPdmBackendVersion = true;
```
Martin Weinelt
e97e82f831