.github, common/travis: run CI without root

this should allow package tests that check things like writability to
succeed and have fewer skipped tests in CI.

- switch to using the `void-LIBC-full` container
- use `uchroot` instead of `ethereal`
- simplify some scripts for this change

common/scripts/lint-conflicts

@@ -7,6 +7,8 @@
 # With single path as argument, read that local repository.
 # With -a flag, find conflicts between packages indexed in xlocate.
 
+set -e
+
 if [ "$#" = 0 ]; then
 	binpkgs="$PWD/hostdir/binpkgs"
 elif [ "$1" = -a ]; then