Downloads bug caused by Pkg test bug

[vtjnash]

It looks like Pkg is messing up the cwd set by the test harness, breaking future tests (such as this downloads.jl failure)

      From worker 2:	ERROR: SystemError: opening file "C:\\buildkite-agent\\builds\\win2k22-amdci6-4\\julialang\\julia-master\\julia-15244663a3\\share\\julia\\stdlib\\v1.11\\Pkg\\test\\download_exec.jl": No such file or directory
      From worker 2:	Stacktrace:
      From worker 2:	  [1] systemerror(p::String, errno::Int32; extrainfo::Nothing)
      From worker 2:	    @ Base .\error.jl:176
      From worker 2:	  [2] systemerror
      From worker 2:	    @ Base .\error.jl:175 [inlined]
      From worker 2:	  [3] open(fname::String; lock::Bool, read::Nothing, write::Nothing, create::Nothing, truncate::Nothing, append::Nothing)
      From worker 2:	    @ Base .\iostream.jl:293
      From worker 2:	  [4] open
      From worker 2:	    @ Base .\iostream.jl:275 [inlined]
      From worker 2:	  [5] open(f::Base.var"#463#464"{String}, args::String; kwargs::@Kwargs{})
      From worker 2:	    @ Base .\io.jl:394
      From worker 2:	  [6] open
      From worker 2:	    @ Base .\io.jl:393 [inlined]
      From worker 2:	  [7] read
      From worker 2:	    @ Base .\io.jl:486 [inlined]
      From worker 2:	  [8] _include(mapexpr::Function, mod::Module, _path::String)
      From worker 2:	    @ Base .\loading.jl:2255
      From worker 2:	  [9] include(mod::Module, _path::String)
      From worker 2:	    @ Base .\Base.jl:526
      From worker 2:	 [10] exec_options(opts::Base.JLOptions)
      From worker 2:	    @ Base .\client.jl:318
      From worker 2:	 [11] _start()
      From worker 2:	    @ Base .\client.jl:525
download                                         (2) |         failed at 2023-12-13T09:43:40.855
Error During Test at C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\testdefs.jl:24
  Got exception outside of a @test
  LoadError: download test failed, cmd : `'C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\bin\julia.exe' -C native '-JC:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\lib\julia\sys.dll' --depwarn=error --check-bounds=yes -g1 --startup-file=no --depwarn=no --startup-file=no download_exec.jl`
  Stacktrace:
    [1] error(s::String)
      @ Base .\error.jl:35
    [2] top-level scope
      @ C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\download.jl:5
    [3] include
      @ .\Base.jl:526 [inlined]
    [4] macro expansion
      @ C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\testdefs.jl:33 [inlined]
    [5] macro expansion
      @ C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\stdlib\v1.11\Test\src\Test.jl:1598 [inlined]
    [6] macro expansion
      @ C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\testdefs.jl:26 [inlined]
    [7] macro expansion
      @ .\timing.jl:513 [inlined]
    [8] runtests(name::String, path::String, isolate::Bool; seed::UInt128)
      @ Main C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\testdefs.jl:24
    [9] runtests
      @ Main C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\testdefs.jl:5 [inlined]
   [10] invokelatest(::Any, ::Any, ::Vararg{Any}; kwargs::@Kwargs{seed::UInt128})
      @ Base .\essentials.jl:957
   [11] (::Distributed.var"#110#112"{Distributed.CallMsg{:call_fetch}})()
      @ Distributed C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\stdlib\v1.11\Distributed\src\process_messages.jl:286
   [12] run_work_thunk(thunk::Distributed.var"#110#112"{Distributed.CallMsg{:call_fetch}}, print_error::Bool)
      @ Distributed C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\stdlib\v1.11\Distributed\src\process_messages.jl:70
   [13] (::Distributed.var"#109#111"{Distributed.CallMsg{:call_fetch}, Distributed.MsgHeader, Sockets.TCPSocket})()
      @ Distributed C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\stdlib\v1.11\Distributed\src\process_messages.jl:286
  in expression starting at C:\buildkite-agent\builds\win2k22-amdci6-4\julialang\julia-master\julia-15244663a3\share\julia\test\download.jl:4

[IanButterworth]

Fixed by #52521