[CI] Include "Save NEYNAR_API_KEY to file" step in [ios/android]_release workflows

Summary:
Pretty much identical to existing "Save ALCHEMY_API_KEY to file" step.

Also included the secret in GH Actions Secret store.

Depends on D11380

Test Plan:
Close reading + matching of existing "Save ALCHEMY_API_KEY to file" step.

Will be tested the next time we make a release.

Reviewers: ashoat, ginsu, varun, will

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11381