fix: run MK on macOS w/ testing infra only

MK is increasingly less important to us. But we know that we have
people running versions of MK compiled by themselves.

These persons mainly run NDT tests.

To have some confidence we're not breaking their workflows, let us
periodically run a MK probe from macOS against the testing infra.

This end-to-end test is not very precise, because we are not running
the same version they are running and maybe it's not even the same
OS and we likely have different dependencies. Yet, it seems a reasonable
best effort at ensuring we ain't breaking things too badly.

See ooni/backend#461 (comment)

.github/workflows/measurementkit.yml

@@ -1,5 +1,13 @@
 # Not very popular among users but yet useful to garner confidence that
 # MK is WAI, modulo possible dependencies issues.
+#
+# As of 2020-11-13, we don't have any significant OONI client running MK
+# but we know that there are many legacy clients running `ndt` using version
+# of MK compiled by people independently of OONI. So we are keeping this
+# workflow for now, to be sure we are not breaking this use case.
+#
+# Because this is increasingly less important to us, we only run tests
+# using OONI's testing infrastructure.
 name: measurementkit
 on:
   pull_request:
@@ -22,6 +30,7 @@ jobs:
             gunzip -f $f
           done
           measurement_kit -o output/mk.jsonl             \
+                --bouncer=https://ams-pg-test.ooni.org/  \
                 --ca-bundle-path ca-bundle.pem           \
                 --geoip-country-path country.mmdb        \
                 --geoip-asn-path asn.mmdb                \