make std.os.getenvW case insensitive
make std.os.getenvW case insensitive

partially addresses #4603

Fixing std.process.getEnvMap is NOT included in this commit.

zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig