chore(ci): test gpu backend on ci

[cocoa-xu]

No description provided.

[cocoa-xu]

Getting some errors in test/emlx/nx_test.exs:

----test/emlx/nx_test.exs----

LIBMLX_ENABLE_DEBUG=false
Terminating due to uncaught exception: [metal::Device] Unable to load kernel gg0_copyint32int32

bash: line 1:  1914 Abort trap: 6           mix test $NAME

Might be an upstream issue because the same error also presents when building from source on CI:

https://github.com/elixir-nx/emlx/actions/runs/13325467453/job/37217649558?pr=75#step:10:23