tweak how Pkg is loaded for precompiling when testing

[KristofferC]

This is to prevent #3442 when moving Pkg out of the sysimage.

[IanButterworth]

hmm I think something's wrong here. On latest julia master

(@v1.11) pkg> test MyFoo
     Testing MyFoo
      Status `/private/var/folders/1z/jf841bdj73bdj3vk7kc7f_3w0000gn/T/jl_yhzuNm/Project.toml`
...
Precompiling project...
        Info Given Pkg was explicitly requested, output will be shown live 
ERROR: LoadError: Replacing module `Pkg`
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] macro expansion
   @ Base ./loading.jl:1847 [inlined]
 [3] macro expansion
   @ Base ./lock.jl:267 [inlined]
 [4] register_root_module(m::Module)
   @ Base ./loading.jl:1841
 [5] include
   @ Base ./Base.jl:493 [inlined]
 [6] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
   @ Base ./loading.jl:2216
 [7] top-level scope
   @ stdin:3
in expression starting at /Users/ian/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/Pkg.jl:3
in expression starting at stdin:3
  ✗ Pkg
  0 dependencies successfully precompiled in 1 seconds. 3 already precompiled.

ERROR: The following 1 direct dependency failed to precompile:

Pkg [44cfe95a-1eb2-52ea-b672-e2afdf69b78f]

Failed to precompile Pkg [44cfe95a-1eb2-52ea-b672-e2afdf69b78f] to "/Users/ian/.julia/compiled/v1.11/Pkg/jl_0iskQe".
ERROR: LoadError: Replacing module `Pkg`
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] macro expansion
   @ Base ./loading.jl:1847 [inlined]
 [3] macro expansion
   @ Base ./lock.jl:267 [inlined]
 [4] register_root_module(m::Module)
   @ Base ./loading.jl:1841
 [5] include
   @ Base ./Base.jl:493 [inlined]
 [6] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
   @ Base ./loading.jl:2216
 [7] top-level scope
   @ stdin:3
in expression starting at /Users/ian/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/Pkg.jl:3
in expression starting at stdin:
Stacktrace:
 [1] pkgerror(msg::String)
   @ Pkg.Types ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/Types.jl:70
 [2] precompile(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; internal_call::Bool, strict::Bool, warn_loaded::Bool, already_instantiated::Bool, timing::Bool, _from_loading::Bool, kwargs::@Kwargs{io::Base.TTY})
   @ Pkg.API ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/API.jl:1626
 [3] precompile(pkgs::Vector{Pkg.Types.PackageSpec}; io::Base.TTY, kwargs::@Kwargs{warn_loaded::Bool})
   @ Pkg.API ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/API.jl:159
 [4] precompile(; name::Nothing, uuid::Nothing, version::Nothing, url::Nothing, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::@Kwargs{warn_loaded::Bool})
   @ Pkg.API ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.11/Pkg/src/API.jl:174
 [5] top-level scope
   @ none:6
     Testing Precompilation of test environment failed. Continuing to tests
     Testing Running tests...