Guard against conda-build not conda

setup.py

@@ -60,16 +60,18 @@ def build_extensions(self):
                 cwd=FASTJET,
             )
 
-            # Hack to inject the required CXXFLAGS and LDFLAGS for building
-            # on macOS aarch64.
+            # Inject the required CXXFLAGS and LDFLAGS for building on
+            # aarch64 macOS. As Homebrew can not be used at any part in a
+            # conda-forge build, guard against this by checking for the
+            # conda-build environment variable.
             # This is a bad hack, and will be alleviated if CMake can be used
             # by FastJet and FastJet-contrib.
             # c.f. https://github.com/scikit-hep/fastjet/issues/310
             if (
                 sys.platform == "darwin"
                 and platform.processor() == "arm"
                 and "HOMEBREW_PREFIX" in os.environ
-                and "CONDA_PREFIX" not in os.environ
+                and "CONDA_BUILD" not in os.environ
             ):
                 os.environ["CXXFLAGS"] = (
                     os.environ.get("CXXFLAGS", "")