Show apt sources and preferences used in the CI builds

This may be useful to understand where do the packages come from.

build/tools/before_install.sh

@@ -11,6 +11,13 @@ SUDO=sudo
 case $(uname -s) in
     Linux)
         if [ -f /etc/apt/sources.list ]; then
+            # Show information about the repositories and priorities used.
+            echo 'APT sources used:'
+            $SUDO grep --no-messages '^[^#]' /etc/apt/sources.list /etc/apt/sources.list.d/* || true
+            echo 'APT preferences:'
+            $SUDO grep --no-messages '^[^#]' /etc/apt/preferences /etc/apt/preferences.d/* || true
+            echo '--- End of APT files dump ---'
+
             run_apt() {
                 echo "Running apt-get $@"