Flaky CI on `master`

[cmichi]

The test cmd::metadata::tests::generate_metadata succeeds sometimes and fails at other times.

These are runs for the same commit:

• Success: https://gitlab.parity.io/parity/cargo-contract/-/jobs/864147
• Failure: https://gitlab.parity.io/parity/cargo-contract/-/jobs/864124

A re-run usually succeeds at some point.

It fails at an assertion where the hash of the generated Wasm on the filesystem is compared against the hash written in the metadata source.hash field.

[ascjones]

Closed by #263