Move the test-environment db configuration out of root's ~/.rpmmacros

My reading of https://docs.docker.com/build/building/variables/#scoping
suggests that we could now remove the second "ARG PREFIX" from the
make stage because it should be inherited in the FROM base thing, but
s*** breaks if I remove it, so we'll just *shrug* and let it be I guess.

This is the first step to fixing rpm-software-management#3521.

tests/Dockerfile.fedora

@@ -63,7 +63,10 @@ RUN rm -f /usr/lib/rpm/macros.d/macros.transaction_ima
 RUN dnf -y --enablerepo=updates install "sequoia-sq >= 1.0"
 RUN dnf clean all
 
-RUN echo "%_dbpath $(rpm --eval '%_dbpath')" > /root/.rpmmacros
+ARG PREFIX
+# Make sure the built rpm find the system db (for shell usage)
+RUN mkdir -p ${PREFIX}/etc/rpm
+RUN echo "%_dbpath $(rpm --eval '%_dbpath')" > ${PREFIX}/etc/rpm/macros.db
 
 # Workaround for pkgconf(1)'s unlisted dependency on rpm.
 # This is needed for cmake to work without an rpm installation.