Fix emscripten as os rather than env.

b7af607 ("Switch to a target structure...") is checking whether the
target environment is emscripten, but it seems emscripten is the OS. Fix
this, which should resolve the issue in
<rust-lang/rust#128691 (comment)>.

build.rs

@@ -55,7 +55,7 @@ fn main() {
     println!("cargo:rustc-cfg=feature=\"unstable\"");
 
     // Emscripten's runtime includes all the builtins
-    if target.env == "emscripten" {
+    if target.os == "emscripten" {
         return;
     }