break inference in Windows env caching to prevent a large set of invalidations from popular packages

[KristofferC]

Alternative to #52758 (see that PR for a description of the issue).

1.9:

julia> @btime ENV["=C:"]
  136.925 ns (5 allocations: 280 bytes)
"C:\\Users\\Kristoffer"

1.10.0:

julia> @btime ENV["=C:"]
  102.345 ns (3 allocations: 200 bytes)
"C:\\Users\\Kristoffer"

This patch:

julia> @btime ENV["=C:"]
  120.081 ns (3 allocations: 248 bytes)
"C:\\cygwin64\\home\\Kristoffer\\julia"