Use same syntax for extra arguments as wasm-pack test

[fkettelhoit]

The syntax for passing extra arguments to wasm-pack build and wasm-pack test has diverged due to #851, with the result that an additional -- is expected by wasm-pack build but not by wasm-pack test:

wasm-pack build -- --features foo
wasm-pack test --features foo

This PR mirrors the change made in #851 and applies it to wasm-pack build so that both commands use the same syntax again (without the need for an extra -- and with the same syntax as cargo build and cargo test):

wasm-pack build --features foo
wasm-pack test --features foo

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