Add PIPENV_SKIP_LOCK to disable locking

[giuscri]

Locking can be very slow on some machines - for example, I always run pipenv with the --skip-lock option.

It would be useful to control that behaviour from an environment variable.

[techalchemy]

It is literally slow only the first run now due to hash caching. This isn’t something I think we want to support or encourage since you are skipping all of the functionality of the tool

[techalchemy]

We might want to revisit this (see #2200)

[pirate]

It is literally slow only the first run now due to hash caching.

FWIW This is definitely not true in my experience, anytime I add a new package locking takes a bunch of extra time. I only lock before pushing to master in preparation for deploys, 90% of my runs during development I have to add --skip-lock.