Simplify rpm build deps install

[cpuguy83]

Right now when we build an rpm we first generate an RPM that has the build deps of the spec as runtime deps and then we install that package which triggers all the deps to be installed.

As it turns out we can generate a source package and use dnf install --builddeps <srcpkg>.rpm
This would in theory save a bit of time since it can skip generating the intermediate package (and intermediate source package!) and should make errors easier to track down.