xls: Use Conda-packaged compiler

[ajelinski]

In xls (currently still in review: #168) there's an issue with using Conda-packaged compiler for building.
Currently, the executables are linked against the OS's glibc and there are certain functions used which require a relatively recent version of it (2.30 which isn't available on Debian Buster or Ubuntu 18.04).
To mitigate the problem, Ubuntu 18.04 will be used to build the package unless the problem with using Conda-packaged compiler is solved.