Update Semaphore CI example with better caching

[bahmutov]

Should install dependencies once and use them in each parallel job

This is the recommended way of achieving this is to install dependencies/store cache in a block and then restore cache and run tests in the parallel jobs. The recipe would be the following:

• download the dependencies in block A
• cache store dependencies in block A
• create a prologue that cache restore the cached dependencies. if using parallelism this prologue will run for every job
• run your tests in the block with parallelism