Nested registry set-up is very slow

[maleadt]

@fredrikekre noticed that the Pkg.Registry.add command executed by julia-buildkite-plugin takes a surprisingly long time when it executed in the sandbox set-up by sandbox-buildkite-plugin. For example, https://buildkite.com/julialang/linearalgebra-dot-jl/builds/9#01934dca-16ee-4a5c-a6d4-dd33fd222768, where the outer registry set-up (Updating the registry) takes 2s, but the inner one takes 1m2s.

It's not clear to me where this overhead is coming from; I couldn't reproduce locally when running in a simple sandbox, but let's file an issue so that this doesn't get lost.