Retry when grpahql-schema-linter gives an error.
Retry when grpahql-schema-linter gives an error.

This works around a bug in the linter:
https://github.com/cjoudrey/graphql-schema-linter/issues/211

Retrying isn't the best solution, but it's definitely the easiest!

Auditors: marksandstrom

Test plan:
I ran `ka-lint services/assignments/schema.graphql` without error in
webapp.  It's fingers-crossed that it solves this problems for others
(I don't see it on linux).

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter