test: prevent rpmbuild from writing to system rpmdb

rpmbuild with default dbpath will write to /var/lib/rpm. Set it to the
temporary directory.

test/data/build-rpms-and-repos.sh

@@ -32,6 +32,7 @@ function build_rpms() {
 
     # Note: _build_name_fmt requires escaped %% for use in headerSprintf()
     rpmbuild -ba \
+        --dbpath "${RPMBUILD_TOPDIR}" \
         --define="_topdir ${RPMBUILD_TOPDIR}" \
         --define="_srcrpmdir ${TARGET_DIR}" \
         --define="_rpmdir ${TARGET_DIR}" \