chore(ios): Cleanup RELEASE_OEM_FIRSTVOICES var in build.sh scripts

[darcywong00]

For addressing #9051 (I realize assigned to @sgschantz - I'm just starting it since I had to implement a similar patterns on Android of checking env vars for building FirstVoices for Android app.

• Fixes unbound variable in tools/prepRelease.sh
• Checks RELEASE_OEM_FIRSTVOICES env var for build targets (in the pattern from fix(android): Require RELEASE_OEM env vars to build FirstVoices Android #13725)

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if possible we should eliminate one of RELEASE_OEM and RELEASE_OEM_FIRSTVOICES env vars.

Currently not handled for any of the FV platforms.

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Test-bot: skip

[keymanapp-test-bot]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• iOS
  • Keyman for iOS (simulator image) - build : all tests passed (no artifacts on BuildLevel "build")
  • FirstVoices Keyboards for iOS (simulator image) - build : all tests passed (no artifacts on BuildLevel "build")
  • FirstVoices Keyboards for iOS (simulator image) (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")
  • Keyman for iOS (simulator image) (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")