Revert "tests.installer: Load profile with -o unset"

I must have made a mistake while testing this, because nounset does
not work on any of the distributions.

This reverts commit 2f0db04da0.

tests/installer/default.nix

@@ -217,16 +217,10 @@ let
         $ssh <<EOF
           set -ex
 
-          # enable nounset while loading the profile
-          # this may or may not work on all distros, depending on the quality of their scripts
-          set -u
-
           # FIXME: get rid of this; ideally ssh should just work.
           source ~/.bash_profile || true
           source ~/.bash_login || true
           source ~/.profile || true
-          set +u
-
           source /etc/bashrc || true
 
           nix-env --version