gitlab-ci: use xargs when running exiftool

This should handle long argument lists better.

.gitlab-ci.yml

@@ -33,7 +33,7 @@ lint:
       }
     - apt-get -qy update
     - apt-get -qy install --no-install-recommends exiftool
-    - exiftool -all= `find metadata/ -name '*.jp*g' -o -name '*.png'`
+    - find metadata/ -name '*.jp*g' -o -name '*.png' | xargs exiftool -all=
     - echo "these images have EXIF that must be stripped:"
     - git --no-pager diff --stat
     - git --no-pager diff --name-only --exit-code || export EXITVALUE=1