Stop discover_repo when reaching the depot

[IanButterworth]

Fixes #4318
(Though that will need to be proven on Base CI)

To not emit that warning, discover_repo must not be returning nothing.

I'm not sure why, but part of the problem is that the depot isn't in the user homedir on this CI machine.

Diff `/cache/build/tester-amdci5-15/julialang/julia-master/tmp/jl_OOzF78/environments/v1.13/Project.toml`
  [7876af07] + Example v0.5.5 [loaded: `/cache/build/tester-amdci5-15/julialang/julia-master/tmp/jl_80Rah8/packages/Example/SUIr0/src/Example.jl` (v0.5.5) expected `/cache/build/tester-amdci5-15/julialang/julia-master/tmp/jl_OOzF78/packages/Example/SUIr0/src/Example.jl` (v0.5.5)]
  [9a3f8284] + Random v1.11.0

So I think this might a logical thing to do that avoids the issue, but doesn't fix the core issue?

[IanButterworth]

I'll merge to get this tried out on Base.