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.

[IanButterworth]

Odd. Just adding the observation that both claim to be adding General to the same depot path, which is also odd? It should exist in the 2nd instance?

Added `General` registry to /cache/julia-buildkite-plugin/depots/01933eb6-fbbe-448a-b4d9-f85a2cf4879d/registries

[maleadt]

It's in a sandboxed environment, so everything is fresh there.

[staticfloat]

It's in a sandboxed environment, so everything is fresh there.

In this case that is correct, but I would argue that we should pass /cache:/cache as a workspace mapping here, as suggested by the sandbox-buildkite-plugin README.

My thought here is that this is purely explained by the registry already being resident in /cache (Which is persistent for the outer sandbox) and the inner sandbox being forced to clone it from scratch. 2s is far too quick for an actual git clone of the registry.