templates/stage-cypress-tests: Prevent race conditions between runners
As the parallel cypress runners use the same repository filesystem to execute tests from, some race conditions related to file system operations might happen which make the tests fail.
As a workaround make each cypress runner execute tests from a copy of the swh-web repository filesystem (without root hidden folders).
Such copy takes a dozen of seconds but it makes tests execution much more stable on Jenkins by avoiding the race conditions mentionned above.
Migrated from D8821 (view on Phabricator)