Keep toolchain while switching mode

[dkaszews]

Steps to repro:

1. Run xmake config -v, it will show toolchain: msvc and mode: release
2. Run xmake config -v --toolchain=gcc, now it is gcc/release
3. Run xmake config -v --mode=coverage, now it is msvc/coverage

Is there a way to avoid this behavior of resetting all options which are not explicitly set? I know I can do xmake config --toolchain=gcc --mode=coverage in a single command, but I am trying to reuse code in CI, where it is more convenient to do it separately.

[waruqi]

see

#5767
#2401
#5041
#2401
#823

[dkaszews]

Thank you, the export/import trick works fine