CI: on RHEL distros, install all RPMs

CI: on RHEL distros, install all RPMs

Commit c334e4e81aad15d3092d7c25038a4d2cf1cbff05 altered this step to only
install RPMs necessary for the test suite at the time. However, an upcoming
commit that enables `dot_builtins` causes graphviz-nox-* to depend on
libgvplugin_webp.so.6 that was not being installed. For simplicity, the present
commit reverts c334e4e81aad15d3092d7c25038a4d2cf1cbff05.

Note that this more closely matches the only reasonable way to use the
CI-produced RPMs: install all of them.

Gitlab: related to #1839