need a way for user-level override of project default output directories

The TOML configs allow specifying the work and output directories, but when a given project hard-codes that users are forced to live with build/output files under their git checkouts.

From user feedback:

>Please provide some way to configure the tool to never dump build/output files into the git checkout (not a command line param, some set-and-forget config that will stop it from ever happening again). The fact that building an image leaves root-owned clutter in my git tree is particularly problematic; nobody should *ever* have to do `sudo git clean`

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_Imported on behalf of @ddstreet_

No file or test is named. Start by locating the TOML configuration loading and the code that resolves work and output directories; determine how a user-level setting should take precedence over project values. Done means users can configure persistent output locations that prevent build files from being written into their git checkouts.

Written by the indexing model from the issue text.