Base.runtests: set a temporary Pkg depot path (#42358)

(cherry picked from commit 2c9e051)

Author: DilumAluthge

base/util.jl

@@ -573,6 +573,7 @@ function runtests(tests = ["all"]; ncores::Int = ceil(Int, Sys.CPU_THREADS::Int
     ENV2["JULIA_CPU_THREADS"] = "$ncores"
     delete!(ENV2, "JULIA_LOAD_PATH")
     delete!(ENV2, "JULIA_PROJECT")
+    ENV2["JULIA_DEPOT_PATH"] = mktempdir(; cleanup = true)
     try
         run(setenv(`$(julia_cmd()) $(joinpath(Sys.BINDIR::String,
             Base.DATAROOTDIR, "julia", "test", "runtests.jl")) $tests`, ENV2))