Correctly set minimum OS value for MetalCompile action
Correctly set minimum OS value for MetalCompile action

Previously when an `--{i,mac,tv,watch}os_minimum_os` flag wasn't set,
the fallback value was the SDK version. This change ensures this value
is taken from the following order:
- the bundling rule's `minimum_os_version` attribute if the action is
  executed by building an Apple bundle target
- the `--{i,mac,tv,watch}os_minimum_os` flag value if set
- the SDK version

bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple