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.