ci: use shared JuliaSpacePhysics workflow

[Beforerr]

Moves the test job to the shared workflow in JuliaSpacePhysics/actions (same versions/OS as before; pre is allowed to fail). The shared workflow pins JULIA_CPU_TARGET on x64 so the restored pkgimage cache stays loadable across runner CPUs.

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	9d656b7...	main / 9d656b7...
DimArray	16.3 ± 1.9 μs	16.5 ± 1.8 μs	0.99 ± 0.16
DimArray (direct)	22.3 ± 1.7 μs	22.5 ± 1.7 μs	0.991 ± 0.11
get_data	4.43 ± 0.3 μs	4.42 ± 0.28 μs	1 ± 0.093
get_data (collect)	6.72 ± 2.6 μs	6.8 ± 2.7 μs	0.988 ± 0.55
time_to_load	3.16 ± 0.072 s	3.15 ± 0.072 s	1 ± 0.033

Memory benchmarks

	main	9d656b7...	main / 9d656b7...
DimArray	0.052 k allocs: 2.28 kB	0.052 k allocs: 2.28 kB	1
DimArray (direct)	0.084 k allocs: 3.45 kB	0.084 k allocs: 3.45 kB	1
get_data	0.032 k allocs: 1.17 kB	0.032 k allocs: 1.17 kB	1
get_data (collect)	0.035 k allocs: 6.87 kB	0.035 k allocs: 6.87 kB	1
time_to_load	0.198 k allocs: 11.4 kB	0.194 k allocs: 11.2 kB	1.02

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

Thanks for integrating Codecov - We've got you covered ☂️