WIP: Specify custom UID for build sandbox in elements

• currently broken on non-bwrap sandboxes, investigating *

This allows you to run all the steps involves in a bst build command with a custom UID and GID. This applies to all the build steps (configure, build, install, strip). We need this because some elements expect a non-root UID, or have different behaviour when run as root compared to a normal user.

• MySQL will not start as root, making testing difficult
• tar's default behaviour changes

I couldn't find any documentation of element configuration options at the moment. The element format doc says "refer to the element specific documentation", but I couldn't find that.