Macmini8,1 for another target that runs host tests (flutter/engine#41179)

This target also runs impeller_unittests that aren't safe to run right
now on Macmini7,1

I missed this in https://github.com/flutter/engine/pull/41159

engine/src/flutter/ci/builders/mac_host_engine.json

@@ -128,7 +128,8 @@
             "drone_dimensions": [
                 "device_type=none",
                 "os=Mac-12",
-                "cpu=x86"
+                "cpu=x86",
+                "mac_model=Macmini8,1"
             ],
             "dependencies": [
                 {"dependency": "goldctl", "version": "git_revision:3a77d0b12c697a840ca0c7705208e8622dc94603"}