Fix linting error

This error slipped in on a branch where the shellcheck lint was
running, but got merged to `master` _after_ the shellcheck lint fix
was applied there — so `master`s linting broke.

This is a trivially correct change, so I'm applying and pushing to
`master`.

actions/run-nextstrain-ci-build/action.yaml

@@ -39,7 +39,7 @@ runs:
         else
           echo "Skipping $DIR build due to one or more missing files."
           for i in nextstrain-pathogen.yaml "$DIR"/Snakefile "$DIR"/build-configs/ci/config.yaml; do
-            [[ -f $i ]] || echo missing $i
+            [[ -f $i ]] || echo missing "$i"
           done
         fi
       shell: bash