ConcurrencyViolationError("lock must be held") during repl shell completions

[ericphanson]

Looks like an issue from #52833 (cc @IanButterworth)

If I go into the shell mode and mash the keyboard (incl. backspace), I get:

./julia --startup-file=no -q
shell> eiaorsntoaUnhandled Task ERROR: ConcurrencyViolationError("lock must be held")
Stacktrace:
 [1] concurrency_violation()
   @ Base ./condition.jl:8
 [2] assert_havelock
   @ Base ./condition.jl:25 [inlined]
 [3] assert_havelock
   @ Base ./condition.jl:48 [inlined]
 [4] assert_havelock
   @ Base ./condition.jl:72 [inlined]
 [5] notify(c::Condition, arg::Any, all::Bool, error::Bool)
   @ Base ./condition.jl:150
 [6] notify (repeats 2 times)
   @ Base ./condition.jl:148 [inlined]
 [7] cache_PATH()
   @ REPL.REPLCompletions ~/julia/usr/share/julia/stdlib/v1.11/REPL/src/REPLCompletions.jl:339
 [8] (::REPL.REPLCompletions.var"#14#17")()
   @ REPL.REPLCompletions ~/julia/usr/share/julia/stdlib/v1.11/REPL/src/REPLCompletions.jl:386

julia> versioninfo()
Julia Version 1.11.0-DEV.1269
Commit 2f60a38ca0 (2024-01-13 15:16 UTC)
Platform Info:
  OS: macOS (arm64-apple-darwin23.0.0)
  CPU: 8 × Apple M1
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 4 default, 0 interactive, 2 GC (on 4 virtual cores)
Environment:
  JULIA_NUM_THREADS = 4
  JULIA_PKG_SERVER_REGISTRY_PREFERENCE = eager

This was on #52883 but I don't think it's related to the PR changes there.