You can define rpmbuild variables with %(cmd) at the top of the spec file. Notice the command is in parenthesis, not curly brackets. An example:
%define whoami %(whoami)
And elsewhere in the spec file, such as a script or the build/install sections, use the variable as normal in the curly brackets like this:
echo "The user that built this is %{whoami}"
The cmd can be anything, including your cat command. Be careful – when another user rebuilds the spec file it may not find the file. So it’ll be preferable to use the %{sourcedir} macro like this:
%define path %(cat %{sourcedir}/path_file)
And make sure that path_file is in the source directory and included as a source in the spec file.