ci: add lint error header in log

Signed-off-by: Tanguy ⧓ Herrmann <tanguy@dagger.io>

ci/cue/lint.cue

@@ -50,7 +50,8 @@ import (
 					git status
 
 					find . -name '*.cue' -not -path '*/cue.mod/*' -print | time xargs -t -n 1 -P 8 cue fmt -s
-					test -z "$(git status -s . | grep -e "^ M"  | grep "\.cue" | cut -d ' ' -f3 | tee /dev/stderr)"
+					modified="$(git status -s . | grep -e "^ M"  | grep "\.cue" | cut -d ' ' -f3 || true)"
+					test -z "$modified" || (echo -e "linting error in:\n${modified}" > /dev/stderr ; false)
 					"""#
 			},
 		]