Define default build type for CMake recipes

[imciner2]

We should set the environment variable CMAKE_BUILD_TYPE to be Release so that CMake will use a release build by default if no build type is specified in the command line (ref: https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html).

[imciner2]

Although that environment variable is apparently only used for CMake 3.22 and later. I guess we could actually modify our toolchain file to detect an unitialized CMAKE_BUILD_TYPE variable and set it instead, e.g.

if(NOT DEFINED CMAKE_BUILD_TYPE)
    set(CMAKE_BUILD_TYPE "Release")
endif()

[giordano]

Makes sense to me to have a default which can be overridden on the command line (in case someone wants a release with a debuginfo build, for example).