stage1: fix cmake regression
stage1: fix cmake regression

- add `None` as a valid CMAKE_BUILD_TYPE
- this is a legitimate setting used by packagers

regression was caused by c6df5deb3450e0d8a2ba449c34a0bd195fbce8ec

zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,andrewrk/zig