fix: cross compiling for windows from linux

#[cfg(not(unix))] applies to the host machine (since that's where the
build script is running) rather than the compilation target. Instead,
environment variables are available to provide the information relevant
to the build target at the build script's runtime.

build.rs

@@ -1,11 +1,13 @@
+use std::env;
+
 fn main() {
     println!("cargo::rerun-if-changed=build.rs");
 
-    #[cfg(not(unix))]
-    generate_bundled_icons();
+    if env::var_os("CARGO_CFG_UNIX").is_none() {
+        generate_bundled_icons();
+    }
 }
 
-#[cfg(not(unix))]
 fn generate_bundled_icons() {
     println!("cargo::rerun-if-changed=cosmic-icons");