Only use --find-links on macOS.

Author: freakboy3742

pyproject.toml

@@ -20,7 +20,6 @@ test_sources = ["tests"]
 
 requires = [
     # Provide a source of binary wheels that aren't provided on PyPI.
-    "--find-links", "./wheels",
     "cryptography",
     "lru_dict",
     "pillow",
@@ -33,6 +32,7 @@ test_requires = [
 
 [tool.briefcase.app.testbed.macOS]
 requires = [
+    "--find-links", "./wheels",
     "rubicon-objc",
     "std-nslog",
 ]