Change `packed_simd_2` to `packed_simd` for feature `simd-nightly`

[Windabove]

packed_simd_2 is no more maintained and will fail to compile with the latest nightly.

error: unrecognized platform-specific intrinsic function: `simd_shuffle2`
...
error: unrecognized platform-specific intrinsic function: `simd_shuffle4`
...
error: unrecognized platform-specific intrinsic function: `simd_shuffle8`
...
error: unrecognized platform-specific intrinsic function: `simd_shuffle16`
...
error: unrecognized platform-specific intrinsic function: `simd_shuffle32`
...
error: unrecognized platform-specific intrinsic function: `simd_shuffle64`

since those simd_shuffle* are removed a while ago. Replacing packed_simd_2 with packed_simd fixes this problem.

For more information: rust-lang/packed_simd#356

[Vrixyz]

This has been fixed upstream dimforge/simba#50 ; I'm keeping it open until the whole dependency chain updates.

This still needs a new fix, upstream tracking issue is: dimforge/rapier#633