CircleCI: improve caching for faster startup time

[antoinerg]

This PR slightly improves the execution time of our CircleCi :

1. I removed the steps to store and restore the cache because they were ineffective (the cache key would never match). We can reintroduce them later if we really want to save off a few seconds out of running npm i.

2. Instead of doing a long git checkout process for every jobs, we pass the content of the plotly.js directory to subsequent jobs after build. This add rougly 5 seconds to the persist_to_workspace step. However, we reduce the startup time of all the following jobs by ~ 18 seconds:
   

This will make running tests in parallel more efficient and save us N * 18 seconds of computation time each run.

[antoinerg]

Notice here we use the checksum of "package-lock.json" but when we save the cache, we use the checksum of "package.json". Those two cache names will never match so this is why I removed this "dead" code.

[etpinard]

Nice find. Great work!

I'm curious though: can you try replacing our npm install step with npm ci, and see if we can 🔪 a few more seconds?

[antoinerg]

I'm curious though: can you try replacing our npm install step with npm ci, and see if we can hocho a few more seconds?

npm ci seems faster: roughly 6 seconds versus 15-18 seconds for npm i. I will add it to the current PR.

[etpinard]

npm ci seems faster: roughly 6 seconds versus 15-18 seconds for npm i

🎉 !

[antoinerg]

Now using npm ci in 98c962d !

[etpinard]

💃 💃

[alexcjohnson]

cc @Marc-Andre-Rivet re: npm ci - nice find @etpinard @antoinerg 🏆