CI actions are testing the wrong code

[carns]

See @mdorier 's comment at #61 (comment).

The CI actions should use spack only for installing mobject dependencies, not for installing mobject itself, so that the actual mobject compilation (and make check) is executed against the correct code. See https://github.com/mochi-hpc/mochi-margo/blob/main/.github/workflows/test.yml for an example of how this is done in the mochi-margo project.

[hyoklee]

@carns, would you please assign this to me?