How can I set default build target for Cargo?

You could use a Cargo configuration file to specify a default target-triple for your project. In your project's root, create a .cargo directory and a config file in it with the following contents:

[build]
target = "wasm32-unknown-unknown"

As listed in the Cargo documentation, you can create a .cargo/config and specify the target:

[build]
target = "my-custom-target"