fix: respect DENO_DIR and XDG_* environment variables for fs cache

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Closes #357. To resolve the "configDir" (now appropriately renamed to "cacheDir");

1. $DENO_DIR is checked, and is the result if present.
2. Then $XDG_CACHE_HOME is checked, if present the result becomes $XDG_CACHE_HOME/deno.
3. Finally defaults to $HOME/.deno if the above two fail.

The path resolved from the above three steps is then joined with the deployctl subdirectory.

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I plan on also PR-ing to Deno core to support XDG lookup there, where it looks like https://github.com/denoland/deno_cache_dir is used - https://github.com/denoland/deno_cache_dir also provides a TypeScript library. I'll leave this open but maybe we should add @deno/cache-dir as a dependency and use that instead of this patch?

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I prefer the solution in #359, but we would need to wait or update @deno/cache-dir dep later when the XDG PR I made for the library is merged.

[ansipunk]

Damn, this gets really ugly really easy, the cache-dir library approach seems a lot more appealing to me.

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We do the same thing in the cache library, with nicer formatting. I can extract some of that into a variable to make it more readable ig, but yeah I prefer the using the library.