flake : update flake.nix (#2270)

When `isx86_32 || isx86_64`, it will use mkl, else openblas

According to
https://discourse.nixos.org/t/rpath-of-binary-contains-a-forbidden-reference-to-build/12200/3,
add -DCMAKE_SKIP_BUILD_RPATH=ON

Fix #2261, Nix doesn't provide mkl-sdl.pc.
When we build with -DBUILD_SHARED_LIBS=ON, -DLLAMA_BLAS_VENDOR=Intel10_lp64
replace mkl-sdl.pc by mkl-dynamic-lp64-iomp.pc