Expose .xctestrun files to test pre-actions

[brentleyjones]

Pre-actions currently run before the .xctestrun file is generated, so they cannot adjust the prepared test configuration before xcodebuild launches.

Run the pre-action after generating the file and pass its path in TEST_XCTESTRUN_FILE. The hook can then update environment variables, arguments, or other XCTest settings. It still receives SIMULATOR_UDID and runs once on either execution path; TEST_XCTESTRUN_FILE is only supplied when using xcodebuild.

Validation:

• buildifier formatting and lint checks passed.
• Runner regression tests passed in a consuming workspace with this checkout supplied through --override_module=rules_apple=.... They exercise edits to the generated test environment and arguments, verify the hook runs once with the simulator ID, and cover direct xctest execution. Simulator and Xcode commands are stubbed in these tests.
• Regenerated API documentation with bazel run //doc:update; bazel test //doc:check_ios passed.