fix: use stable21 branch for cypress tests

On CI we want to test this with the stable21 branch of the server.
Also make sure to use the specified node version.

For running local the server image will use the BRANCH environment variable
to determine which branch of the server to checkout:
https://github.com/nextcloud/docker-ci/blob/master/server/initnc.sh#L3-L12

Signed-off-by: Azul <azul@riseup.net>

.github/workflows/cypress.yml

@@ -18,7 +18,7 @@ jobs:
         # containers: [1, 2, 3]
         php-versions: [ '7.4' ]
         databases: [ 'sqlite' ]
-        server-versions: [ 'master' ]
+        server-versions: [ 'stable21' ]
 
     steps:
       - name: Checkout server

cypress/Dockerfile

@@ -2,6 +2,7 @@ FROM nextcloudci/server:server-17
 
 RUN mkdir /var/www/html/data
 RUN chown -R www-data:www-data /var/www/html/data
+ENV BRANCH stable21
 
 ENTRYPOINT /usr/local/bin/initAndRun.sh