Introduce DFX_CACHE_ROOT instead of using DFX_CONFIG_ROOT for the cache directory

[paulyoung]

Introducing a new DFX_CACHE_ROOT environment variable would address the issue I describe in paulyoung/nixpkgs-dfinity-sdk#4.

I imagine the change involves changing the following code, falling back to DFX_CONFIG_ROOT for DFX_CACHE_ROOT for backwards compatibility, and adding some new tests.

sdk/src/dfx/src/config/cache.rs

Lines 69 to 70 in d10f20b

	pub fn get_cache_root() -> DfxResult<PathBuf> {
	let config_root = std::env::var("DFX_CONFIG_ROOT").ok();

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I'd like to make a PR but can't because of #1955:

$ nix-shell --show-trace                                                         ~/projects/dfinity/sdk/main
ERROR: Repository not found.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error: program 'git' failed with exit code 128

       … while fetching the input 'git+ssh://[email protected]/dfinity-lab/common?ref=refs%2fheads%2fmaster&rev=da56c3d8094a0f2e18533b529ecb4eb8804927d1'

[paulyoung]

I noticed that my machine has both ~/.cache/dfinity/versions/ and ~/.config/dfx/ directories.

I'm not sure if these are from an older version of the SDK but they still seem appropriate and further support having DFX_CACHE_ROOT.