Consider using method deletion instead of custom sysimage

[KristofferC]

Right now, we use a custom sysimage because loading a second copy of LinearAlgebra causes ambiguities due to piracy. However, we should be able to just delete all the existing LinearAlgebra methods with e.g.

Base.visit(Core._.defs) do m; nameof(m.module) === :LinearAlgebra && Base.delete_method(m); end

(courtesy of @vtjnash) and then load the second copy of LinearAlgebra, perhaps with

Base.unreference_module(Base.PkgId(Base.UUID("37e2e46d-f89d-539d-b4ee-838fcccc9c8e"), "LinearAlgebra"))

That would save a lot of time in CI.

[KristofferC]

This is done