EnvHash should be immutable

This is a bit of a relic, and was never really updated to be a standard immutable singleton. I doubt there is a lot of practical difference, but hey.

FYI this type so old, it is now the *second* time it is called a `struct`. :)

base/env.jl

@@ -63,7 +63,7 @@ end # os test
 
 A singleton of this type provides a hash table interface to environment variables.
 """
-mutable struct EnvHash <: Associative{String,String}; end
+struct EnvHash <: Associative{String,String}; end
 
 """
     ENV