framework, unotest: do not override UserInstallation from environment

The PythonTest_dbaccess_python may fail in incremental builds because of
stale JVM path in workdir/unittest/user/config/javasettings_Linux_X86_64.xml
so it's better to use the fresh UserInstallation that is already passed
in from PythonTest.mk.  To make that work, tolerate non-existing path in
SubstitutePathVariables::SetPredefinedPathVariables() at least when
running in the build environment, which is probably better than
effectively running with "/" as UserInstallation anyway...

This partially reverts commit 6dbb6275ebd1a4299099c3b6bc82ec4ee0e1fb86.

Change-Id: I4d42d53c049056590662828360c4ce5a29af0f4b