pw_env_setup: Fix arm64 Mac Python

Bug: 458
Change-Id: I054ff164c0a9cdfd6e4a04854db513983af7afaa
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/58401
Reviewed-by: Michael Spang <spang@google.com>
Reviewed-by: Scott James Remnant <keybuk@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
diff --git a/pw_env_setup/py/pw_env_setup/cipd_setup/python.json b/pw_env_setup/py/pw_env_setup/cipd_setup/python.json
index da67b78..8a71eb4 100644
--- a/pw_env_setup/py/pw_env_setup/cipd_setup/python.json
+++ b/pw_env_setup/py/pw_env_setup/cipd_setup/python.json
@@ -6,11 +6,20 @@
       "linux-amd64",
       "linux-arm64",
       "mac-amd64",
-      "mac-arm64",
       "windows-amd64"
     ],
     "tags": [
       "version:2@3.9.5.chromium.19"
     ]
+  },
+  {
+    "_comment": "TODO(pwbug/455) Use 3.9 for Macs too.",
+    "path": "infra/3pp/tools/cpython3/${platform}",
+    "platforms": [
+      "mac-arm64"
+    ],
+    "tags": [
+      "version:2@3.8.10.chromium.21"
+    ]
   }
 ]