Fix the browser node and update demo script (#5613)

* Fix browser node

* Fix readme

substrate/bin/node/cli/browser-demo/README.md

 # How to run this demo
 
 ```sh
-cargo install wasm-pack		# If necessary
-
-wasm-pack build --target web --out-dir ./browser-demo/pkg --no-typescript --release ./.. -- --no-default-features --features "browser"
-
-xdg-open index.html
+cargo install wasm-bindgen		# If necessary
+./build.sh
 ```

substrate/bin/node/cli/browser-demo/build.sh

 #!/usr/bin/env sh
-wasm-pack build --target web --out-dir ./browser-demo/pkg --no-typescript --release ./.. -- --no-default-features --features "browser"
+cargo +nightly build --release -p node-cli --target wasm32-unknown-unknown --no-default-features --features browser -Z features=itarget
+wasm-bindgen ../../../../target/wasm32-unknown-unknown/release/node_cli.wasm --out-dir pkg --target web
 python -m http.server 8000

substrate/utils/browser/src/lib.rs

@@ -49,7 +49,7 @@ where
 		format!("{} (Browser)", name),
 		"unknown",
 		Default::default(),
-		&std::env::current_dir().expect("current directory must exist"),
+		&std::path::PathBuf::new(),
 	);
 	network.boot_nodes = chain_spec.boot_nodes().to_vec();
 	network.transport = TransportConfig::Normal {