CP-13121 Add stable to the ppas mirror URL in the linux-pkg

PR URL: https://www.github.com/delphix/linux-pkg/pull/381

Author: SumoSourabh

setup.sh

@@ -80,7 +80,7 @@ function configure_apt_sources() {
 		deb ${primary_url} ${UBUNTU_DISTRIBUTION}-backports main restricted universe multiverse
 		deb-src ${primary_url} ${UBUNTU_DISTRIBUTION}-backports main restricted universe multiverse
 
-		deb ${secondary_url} ${UBUNTU_DISTRIBUTION} main multiverse universe
+		deb ${secondary_url} ${UBUNTU_DISTRIBUTION} main multiverse universe stable
 		EOF" || die "/etc/apt/sources.list could not be updated"
 
 	logmust sudo apt-key add "$TOP/resources/delphix-secondary-mirror.key"