Comprehensive tests for shared builds

Comprehensive tests for shared builds

Added `shared-build` driver

Added `build_mode` key to the Python driver

Fix config generation in `alr update` in shared mode

Remove need for compiler in absence of dependencies

Fixed monorepo tests and base dir

Delay config generation in sandboxed mode too

Include explicit switches in build hash inputs

This is necessary because for shared builds we're never regenerating the
configuration, so changes in switches went undetected

Require complete config for sandboxed build

Self-review