feat: Introduce DFX_CACHE_ROOT environment variable

[paulyoung]

Description

A new environment variable, DFX_CACHE_ROOT, has been introduced to allow setting the cache root directory to a different location than the configuration root directory. Previously DFX_CONFIG_ROOT was repurposed for this which only allowed one location to be set for both the cache and configuration root directories.

The motivation for this is described in paulyoung/nixpkgs-dfinity-sdk#4, where the cache root directory is necessarily set to a read-only directory. This means that the configuration root directory is also read-only, resulting in the error Cannot create config directory when trying to write .config/dfx

Fixes #2106 and paulyoung/nixpkgs-dfinity-sdk#4

How Has This Been Tested?

Only tested with cargo test locally. Unable to run end-to-end tests locally due to #1955.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[paulyoung]

Please advise on the PR title. The check failed but I used the format shown at https://www.conventionalcommits.org/en/v1.0.0/#commit-message-with--to-draw-attention-to-breaking-change

[sesi200]

Hi Paul

Thank you very much for your PR! We would like add those changes. Annoyingly, Hydra (required part of CI, removing it is WIP) does not properly work on external PRs, so someone from the team will have to re-implement your changes and create a separate PR to merge them. This is planned for the next sprint. We'll keep you posted on the progress.

Severin

[ericswanson-dfinity]

@paulyoung Thanks for this! The CI (internal, that you can't see) is failing in this test, when it tries to write to $HOME/.cache: https://github.com/dfinity/sdk/blob/master/src/dfx/src/lib/environment.rs#L490 .

[2022-03-31 08:37:17] thread 'lib::environment::tests::test_passwords' panicked at
 'called `Result::unwrap()` on an `Err` value: Cannot create cache directory at '/homeless-shelter/.cache/dfinity'.',
 src/dfx/src/lib/environment.rs:501:14

If you'd like to try changing this test to alter DFX_CACHE_ROOT rather than DFX_CONFIG_ROOT, it's possible that the tests will pass, and we can get this merged in right away. Otherwise I will take care of it in our next sprint.

[ericswanson-dfinity]

@paulyoung So sorry, but it looks like there is one more place in the internal CI that needs to be changed from DFX_CONFIG_ROOT to DFX_CACHE_ROOT, here: https://github.com/dfinity/sdk/blob/master/check-binaries.nix#L18

[paulyoung]

Please could we get a release that includes this change?