Properly isolate sysimg bootstrap environment (#50683)

The stdlib environment should be isolated from the global environment
while it is bootstrapping; it should only load from the stdlib
directory.

Author: staticfloat

base/sysimg.jl

@@ -77,6 +77,7 @@ let
 
     tot_time_stdlib = 0.0
     # use a temp module to avoid leaving the type of this closure in Main
+    push!(empty!(LOAD_PATH), "@stdlib")
     m = Module()
     GC.@preserve m begin
         print_time = @eval m (mod, t) -> (print(rpad(string(mod) * "  ", $maxlen + 3, "─"));