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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	2c54ff1...	main / 2c54ff1...
elx/full_load	19.6 ± 1.4 μs	19.5 ± 1.4 μs	1.01 ± 0.1
elx/sum_array	1.75 ± 1.2 μs	1.66 ± 1.7 μs	1.05 ± 1.3
elx/sum_lazy	1.79 ± 1.8 μs	1.71 ± 1.7 μs	1.05 ± 1.5
elx/sum_var_access	1.72 ± 0.42 μs	1.72 ± 0.66 μs	0.998 ± 0.45
elx/var_access	0.191 ± 0.002 μs	0.19 ± 0.003 μs	1.01 ± 0.019
mms/full_load	12 ± 1.2 μs	11.2 ± 1.3 μs	1.07 ± 0.16
mms/sum_array	0.0717 ± 0.0046 s	0.0683 ± 0.0057 s	1.05 ± 0.11
mms/sum_slice	2.48 ± 0.1 ms	2.48 ± 0.24 ms	0.998 ± 0.1
mms/sum_var_access	2.53 ± 0.23 ms	2.48 ± 0.24 ms	1.02 ± 0.14
mms/var_access	0.068 ± 0.003 μs	0.067 ± 0.001 μs	1.01 ± 0.047
time_to_load	0.121 ± 0.00055 s	0.12 ± 0.00043 s	1.01 ± 0.0058

Memory benchmarks

	main	2c54ff1...	main / 2c54ff1...
elx/full_load	0.217 k allocs: 24.8 kB	0.217 k allocs: 24.8 kB	1
elx/sum_array	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_lazy	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_var_access	14 allocs: 28.3 kB	14 allocs: 28.3 kB	1
elx/var_access	1 allocs: 0.234 kB	1 allocs: 0.234 kB	1
mms/full_load	0.065 k allocs: 6.45 kB	0.065 k allocs: 6.45 kB	1
mms/sum_array	1.04 k allocs: 31.7 MB	1.04 k allocs: 31.7 MB	1
mms/sum_slice	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/sum_var_access	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/var_access	1 allocs: 0.219 kB	1 allocs: 0.219 kB	1
time_to_load	0.2 k allocs: 11.8 kB	0.2 k allocs: 11.8 kB	1