Specify working directory for the serve-locally target

Meson doc says that run_targets are run in an unspecified directory.

meson.build

@@ -632,6 +632,7 @@ if host_platform == 'emscripten'
 		command: [
 			python3_prog,
 			serve_wasm_py,
+			'@BUILD_ROOT@',
 		],
 		depends: powder_exe,
 	)

resources/serve-wasm.template.py

@@ -7,6 +7,13 @@ import ssl
 import sys
 import time
 
+(
+	script,
+	build_root,
+) = sys.argv
+
+os.chdir(build_root)
+
 HTTP_HOST  = '127.0.0.1'
 HTTP_PORT  = 8000
 HTTP_INDEX = 'serve-wasm.index.html'