clang uses considerably less memory on Linux allowing more concurrent instances than with g++.
Using clang reduced build times on a machine from 63m to 32m.
pixi sets a variety of environment variables, which unfortunately includes compiler optimization flags.
This PR unsets the compiler flags for parity with other environments.
A consequence of this is that debug builds are no longer build with high optimization, which is a desirable outcome.
Windows requires running from an installed FreeCAD, not simply the build environment. This
commit performs the install and execution from the installation environment.
