fix: use toFilePath in package config Uri (#2262)

fixes #2231

Thanks to @d-markey
#2231 (comment)
for figuring it out.

This fix uses the same technique as in precompiled tests:

https://github.com/dart-lang/test/blob/9fbbfdbee18a686e3f84a386a01960ea0543ba01/pkgs/test/test/runner/precompiled_test.dart#L54

Author: Zekfad

pkgs/test_core/lib/src/runner/wasm_compiler_pool.dart

@@ -37,7 +37,7 @@ class WasmCompilerPool extends CompilerPool {
         'compile',
         'wasm',
         '--enable-asserts',
-        '--packages=${(await packageConfigUri).path}',
+        '--packages=${(await packageConfigUri).toFilePath()}',
         for (var experiment in enabledExperiments)
           '--enable-experiment=$experiment',
         '-O0',