refactor: keep old behavior for future testing

tests/conftest.py

@@ -212,12 +212,35 @@ def variants_conda_build_sysroot(monkeypatch, request):
     if not on_mac:
         return {}
 
-    monkeypatch.setenv(
-        "MACOSX_DEPLOYMENT_TARGET",
-        os.path.basename(os.environ["CONDA_BUILD_SYSROOT"])
-        .replace("MacOSX", "")
-        .replace(".sdk", ""),
-    )
+    # if we do not speciy a custom sysroot, we get what the
+    # current SDK has
+    if "CONDA_BUILD_SYSROOT" not in os.environ:
+        monkeypatch.setenv(
+            "CONDA_BUILD_SYSROOT",
+            subprocess.run(
+                ["xcrun", "--sdk", "macosx", "--show-sdk-path"],
+                check=True,
+                capture_output=True,
+                text=True,
+            ).stdout.strip(),
+        )
+
+        mdt = subprocess.run(
+            ["xcrun", "--sdk", "macosx", "--show-sdk-version"],
+            check=True,
+            capture_output=True,
+            text=True,
+        ).stdout.strip()
+    else:
+        # custom sysroots always have names like MacOSX<version>.sdk
+        mdt = (
+            os.path.basename(os.environ["CONDA_BUILD_SYSROOT"])
+            .replace("MacOSX", "")
+            .replace(".sdk", "")
+        )
+
+    monkeypatch.setenv("MACOSX_DEPLOYMENT_TARGET", mdt)
+
     return request.param