This fixes an issue where we'd do logic with Windows-style paths on macOS, and that's
never the right thing to do.
For the LinkNativeCode task, this would manifest as this error when building from windows:
> ld: file too small (length=0) file 'obj/Debug/net6.0-ios/iossimulator-x64/nativelibraries/libSystem.Native.dylib' for architecture x86_64
because the 'ShouldCopyToBuildServer' method would return incorrect results.
For the Codesign task, it would manifest as an exception trying to create a
directory with an empty string (because the directory name of a windows-style
path is an empty string on macOS).
Since this exception was quite useless (just getting the exception message
didn't tell me much about what caused the exception, because it had no stack
trace information), I've also improved error reporting in both of these tasks.
233a0c4505