Correctly handle RPM _prefix macro.

This small change is to correctly handle the RPM macros without changing any of the current deployment defaults/strategies (e.g. /usr).

You can influence the _prefix when running the rpmbuild command:

$ rpmbuild -ba SPECS/jacamar-ci.spec \
    --define "_version 0.4.0.pre" \
    --define "_prefix /ecp/example"

...

$ rpm -qlp jacamar-ci-0.4.0.pre.el7.x86_64.rpm 
/ecp/example/bin/jacamar
/ecp/example/bin/jacamar-auth

Also for gitlab-runner-ecp:

$ rpmbuild -ba SPECS/gitlab-runner-ecp.spec \
    --define "_version 13.7.0" \
    --define "_sha256sum 543d7849d887fcfdabb22ef768e720d34d558fbc0c73860849ceb042b3927b50" \
    --define "_prefix /usr/local" \
    --undefine="_disable_source_fetch"

...

$ rpm -qlp gitlab-runner-ecp-13.7.0-1.el7.x86_64.rpm 
/etc/gitlab-runner
/usr/local/bin/gitlab-ci-multi-runner
/usr/local/bin/gitlab-runner
/usr/local/lib64/gitlab-runner
/usr/local/lib64/gitlab-runner/gitlab-runner