build: migrate to obuild

[OskarLebuda]

Switch out from unbuild to obuild

Benchmark 1: pnpm build:unbuild
  Time (mean ± σ):      2.628 s ±  0.033 s    [User: 3.466 s, System: 0.452 s]
  Range (min … max):    2.579 s …  2.676 s    10 runs

Benchmark 2: pnpm build:obuild
  Time (mean ± σ):      1.990 s ±  0.013 s    [User: 2.542 s, System: 0.338 s]
  Range (min … max):    1.970 s …  2.012 s    10 runs

Summary
  pnpm build:obuild ran
    1.32 ± 0.02 times faster than pnpm build:unbuild