<feat>env: add VFOX_* environment variables

[planktonzp]

Add VFOX_* environment variables to change the location of cache/temp/plugin directory

[planktonzp]

@aooohan @bytemain could you review this PR? thx

[aooohan]

Thanks for the first contribution. However, allowing users to modify these directory paths through environment variables may not convenient. And I think this is not a necessary feature either.

[planktonzp]

Thanks for the first contribution. However, allowing users to modify these directory paths through environment variables may not convenient. And I think this is not a necessary feature either.

However, if it is placed directly under $HOME and cannot be modified, there will be problems with polluting the directory structure. Some users (such as me) have very limited directory space. At the same time, maybe use XDG_* can be more standardized

Anyway, thanks for taking the time to review.