Make RustiCL default for non-apples

[jenshannoschwalm]

As rusticl

• has been proven to be rock-stable for darktable if available
• shows no significant performance penalties (vs ROCm on AMD graphics)
• if by far easier to install than ROCm
• had far less reported issues with driver versions

we favor this as default over ROCm.

[jenshannoschwalm]

Building fails for ... not related to this PR.

In file included from D:/a/darktable/darktable/src/src/imageio/imageio_heif.c:23:
D:/a/_temp/msys64/ucrt64/include/libheif/heif_properties.h:338:56: error: unknown type name 'heif_bad_pixel'
  338 |                                                  const heif_bad_pixel* bad_pixels);
      |                                                        ^~~~~~~~~~~~~~
[101](https://github.com/darktable-org/darktable/actions/runs/26208924120/job/77114920258?pr=21083#step:6:1020)

[TurboGit]

Thanks!