Replace CMAKE_TOOLCHAIN_FILE with CMAKE_PROJECT_INCLUDE_BEFORE #990
Description
Hypothesis: CMake toolchain files are not the right place to customise the build; they are for configuring toolchains only. There are other, more appropriate ways to vary builds in the way that the workflows require.
Test this hypothesis by replacing the toolchain files with something more suitable. Setting one of the PROJECT_INCLUDE variables seems like one possible avenue of exploration.
Acceptance criteria: the end result is that the pipeline runs as it does currently, but with little or no reliance on CMake toolchain files.