Fix `yarn server schema:export` command via makeSchema

[jamesallain]

Description

Fix schema-export. I had to change the script to run it. cross-env wasn't working for me.

Performance impact

none

Security impact

unknown

Checklist

• [x] My code matches the project's code style and yarn lint:fix passes.
• [ ] I've added tests for the new feature, and yarn test passes.
• [ ] I have detailed the new feature in the relevant documentation.
• [ ] I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• [ ] If this is a breaking change I've explained why.

[jamesallain]

I think it's failing where You must set the I_SPONSOR_GRAPHILE environmental variable to the shared secret to use this pre-release software.