Fix web-sys build on some Windows builds

The official pathname separator on Windows is `\` instead of `/`, but
we've been unconditionally using `/`. This typically works on Windows
because Cargo's default `OUT_DIR` listing is a normal `C:\...` path
which works with either `/` or `\`. If, however, a user sets
`CARGO_TARGET_DIR` to a UNC-style path like `\\?\C:\...` then `/` is
*not* the same as `\`, but rather `/` is interpreted as part of the file
name (to allow file names with `/` in the name).

Let's bypass all this and just use a build script output env var.

Closes #943

crates/web-sys/src/lib.rs

@@ -27,4 +27,4 @@ pub fn window() -> Option<Window> {
     js_sys::global().dyn_into::<Window>().ok()
 }
 
-include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
+include!(env!("BINDINGS"));