Fix the flaky nix-profile test

Exclude the `error (ignored)` from the message that is checked by the
install conflict test.

Fix https://github.com/NixOS/nix/issues/8140

tests/nix-profile.sh

@@ -144,6 +144,7 @@ expect 1 nix profile install $flake2Dir
 diff -u <(
     nix --offline profile install $flake2Dir 2>&1 1> /dev/null \
         | grep -vE "^warning: " \
+        | grep -vE "^error \(ignored\): " \
         || true
 ) <(cat << EOF
 error: An existing package already provides the following file: