Set data packages variables only for build/test environment

The way data packages environment variables were propagate to downstream projects was breaking relocatability of builds, in particular for nightly builds and satellite projects.

With this change the data packages environment is only set for build and tests, but not propagated to the .xenv file as anyway lb-run takes care of actual lookup and environment of data packages for the runtime.

The only doubt I have is that an old style cmake project will not be able to inherit the data package environment from a new style upstream project, but it's not so clear if it is a relevant use case (the trivial workaround is to modify the old project to depend on the data package, which is anyway a good idea).

@cattanem, this fixes (in principle) the issue you reported about the wrong $PRCONFIGROOT.