Shard the UI OS and UI EE Cypress jobs #9006

The UI EE job takes about 11 minutes and UI OS about 8 to 11 minutes, with all specs running one after another on a single runner. Once #8999 brings the backend and Postgres jobs down to about 6 minutes, these become the slowest jobs in the Tests workflow.

Local runs on 4 CPUs (the size of the GitHub runner):

Proposal

• Split each job with a matrix and cypress-split (EE 3 shards, OS 2 shards), so new specs are assigned to a shard automatically.
• Cache npm and the Cypress binary, since each shard repeats the setup.
• Expected: both jobs at about 5 minutes, bringing the whole Tests run to about 6 minutes.

Done when

• Each shard passes 5 runs in a row on the PR with no Cypress retries.