Make mx-tester have consistent use of yarn. (#710)

CI was broken because of this shite https://github.com/nodejs/corepack/issues/612

mx-tester.yml

@@ -19,7 +19,7 @@ up:
       127.0.0.1:8081:80 -v $MX_TEST_CWD/test/nginx.conf:/etc/nginx/nginx.conf:ro
       -d nginx
     - corepack yarn install
-    - corepack npx ts-node src/appservice/cli.ts -r -u
+    - corepack yarn ts-node src/appservice/cli.ts -r -u
       "http://host.docker.internal:9000"
     - cp draupnir-registration.yaml $MX_TEST_SYNAPSE_DIR/data/
   after: