tweak how Pkg is loaded for precompiling when testing

src/Operations.jl

@@ -1653,10 +1653,13 @@ end
 function gen_test_precompile_code(source_path::String; coverage, julia_args::Cmd, test_args::Cmd)
     # Note that we cannot load the dev-ed Pkg here during Pkg testing
     # so the `Pkg.precompile` that is run here is the one in the sysimage
+    pkgdir = joinpath(@__DIR__, "..")
     code = """
-        Pkg = Base.require(Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg"))
-        $(Base.load_path_setup_code(false))
-        append!(empty!(ARGS), $(repr(test_args.exec)))
+        pushfirst!(LOAD_PATH, $(repr(pkgdir)))
+        try using Pkg
+        catch e
+            @warn "Pkg not loaded, skipping precompilation."
+        end
         Pkg.precompile(warn_loaded = false)
         """
     return gen_subprocess_cmd(code, source_path; coverage, julia_args)