configure .cache via $XDG_CACHE_DIR

[bukzor]

Forked from #1029

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a) A very convenient, and standard, way to configure tools' cache location is to set $XDG_CACHE_DIR. Many of my project's fixtures do this for other reasons already. Similarly, the most convenient, and standard, way to set "basetmp" would be $TMPDIR.

b) "something a CI system can easily pick up" is exactly these environment variables. Many systems will already support this, and those that don't will support injecting environment variables.

c) "non-containered ci" will either support ~/.cache correctly, or set $XDG_CACHE_DIR, because this is necessary for other tools that use the standard. Again, this is already handled in many of my projects because of other tools.