rpmbuild, backend, frontend: use shlex.quote instead of pipes.quote
    
    Because this deprecation error pollutes the stdout, backend can't read
    the copr-rpmbuild PID and fail the build.
    
        Backend process error: copr-rpmbuild returned invalid PID on
        stdout: /usr/bin/copr-rpmbuild:12: DeprecationWarning: 'pipes' is
        deprecated and slated for removal in Python 3.13
          import pipes
        186
    
    While at it, I am replacing all the occurrences of `pipes`.
    
    Merges: #2371