Improve LIBASSERT_DESIRED_CXX_STANDARD setup #430

Job	Run time
test-linux-fetchcontent	36s
test-linux-add_subdirectory	37s
test-linux-findpackage	23s
test-macos-add_subdirectory	41s
test-macos-findpackage	32s
test-macos-fetchcontent	35s
test-mingw-add_subdirectory	2m 28s
test-mingw-findpackage	1m 51s
test-windows-add_subdirectory	1m 57s
test-mingw-fetchcontent	2m 22s
test-windows-fetchcontent	1m 56s
test-windows-findpackage	2m 19s
	16m 17s

Provide feedback