wasm-builder: Improve workspace handling (#10700)

When building a wasm binary from a different repo inside a local workspace, we did not used the
correct `Cargo.toml` to find the correct patches and features. The solution to this is to just walk
up from the target directory until we find the workspace we are currently compiling. If this
heuristic isn't working, we print a warning and let the user set an env variable
`WASM_BUILD_WORKSPACE_HINT` to tell the `wasm-builder` where the actual workspace is.