log stdout/err from updates check on Debian

This is especially useful for diagnosing update check issues on
Debian/Whonix.

package-managers/upgrades-installed-check

@@ -24,7 +24,7 @@ elif [ -e /etc/debian_version ]; then
     set -e
     set -o pipefail
     # shellcheck disable=SC2034
-    apt_get_update_output="$(apt-get -q update 2>&1)"
+    apt_get_update_output="$(apt-get -q update 2>&1 | tee /proc/self/fd/2)"
     apt_get_upgrade_output="$(LANG="C" apt-get -s upgrade 2>&1)"
     exit_code="$?"
     # shellcheck disable=SC2266