[MLModelDownloader] Disable keychain-dependent tests on macCatalyst and macOS

[ncooke3]

Context

• Intentionally caused by [Keychain] Prevent keychain access from prompting user for permissions on macOS google/GoogleUtilities#75
  • MLModelDownloader is affected because it depends on Installations (which depends on GoogleUtilities's keychain abstraction)
• While we do lose some coverage on CI, GoogleUtilities/#75 makes the macOS keychain behave like the iOS style keychain (which is tested on CI).
• Unblocks some of the CI in Update versions for Release 9.6.0 #10145

Future work

• Open a PR to update MLModelDownloader CHANGELOG with note mentioning new macOS constraint (need to use Keychain Sharing capability) [Edit: See [v9.6.0] Update CHANGELOGs for macOS keychain fix #10181]

#no-changelog

[paulb777]

I wish GitHub had an ignore blank changes options....

LGTM on green CI