Allow caching

[dz0ny]

I was using devenv as flake library but later switched to cli, only noticing that is magnitude slower. What is the main reason for disabling caching?

Without caching:

time devenv shell exit
• Building shell ...
✔ Building shell in 5.1s.
0.76s user 0.18s system 13% cpu 7.154 total

With caching:

time devenv --eval-cache shell exit
• Building shell ...
✔ Building shell in 1.5s.
0.06s user 0.03s system 5% cpu 1.645 total

[domenkozar]

The main reason is that it brings a lot of complexity and bugs, like caching evaluation errors.

[dz0ny]

Ah, well I needed this for CI. Otherwise it was bit too slow.