Check variables in pre-exit hook

[maleadt]

Otherwise a well-timed cancellation can make the pre-exit hook fail:

/etc/buildkite-agent/plugins/github-com-JuliaCI-julia-buildkite-plugin-v1/hooks/pre-exit: line 24: JULIA_DEPOT_PATH: unbound variable

[maleadt]

An example of this: https://buildkite.com/julialang/gpuarrays-dot-jl/builds/481#0185c443-eb1f-4b77-b8cf-e10253c20c8c, where updating the registry failed, resulting in the the pre-exit hook failing too.