Do not generate build artifacts under target for check

[cmichi]

Our documentation for cargo contract check:

/// Check that the code builds as Wasm;
/// does not output any build artifact to the top level `target/` directory

I found that despite this comment we actually output to the target dir. This PR changes this behavior by making check build into a tmp dir.

[ascjones]

Rather than building into a tmp dir, did you consider running cargo check instead of cargo build when invoking cargo for wasm and metadata gen?

e.g. I just ran the raw command cargo check --target wasm32-unknown-unknown -Zbuild-std -Zbuild-std-features=panic_immediate_abort --no-default-features and it worked as expected.

[cmichi]

@ascjones Thanks for the comments, I applied all of them. Can you take another look please?

[ascjones]

LGTM