(#14138) Modify apt::ppa's update-apt exec to use the ${apt::params::provider} parame...

(#14138) Modify apt::ppa's update-apt exec to use the ${apt::params::provider} parameter rather than explicitly calling aptitude.
Previously the update-apt exec would always use /usr/bin/aptitude, which is not necessarily present. This change makes it use ${apt::params::provider} which defaults to /usr/bin/apt-get. This also adds some consistency so that ${apt::params::provider} is used everywhere.