fix: rename Sdk.build -> Sdk.sdl_build to prevent build.zig's build f…

[dwclake]

The build.zig's Sdk.build field has a naming clash with build.zig's Sdk.build() function. I renamed the Sdk.build field to Sdk.sdl_build to prevent this naming clash.

As per Action: Continuous Build #1722

Run zig build install
/home/runner/work/SDL.zig/SDL.zig/build.zig:12:8: error: duplicate struct member name 'build'
/home/runner/work/SDL.zig/SDL.zig/build.zig:126:1: note: duplicate name here
/home/runner/work/SDL.zig/SDL.zig/build.zig:1:1: note: struct declared here
Error: Process completed with exit code 2.

[ikskuh]

Resolved by #199