[3.14] gh-146446: Miscellaneous improvements to iOS XCframework build script (GH-146447) (#146496)

Modifies the iOS build script so that the clean target is more selective about
what is cleaned, the test target has a valid fallback value for ci mode, and the
cross-build directory can be customised.
(cherry picked from commit ca6dfa0f31132c80aaad40855087c2d931dc2d0f)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>