Add flake build

Checks have been disabled (doCheck = false) because they don’t seem to work:

       > failures:
       >
       > ---- commands::vault::tests::test_encrypt_decrypt stdout ----
       >
       > thread 'commands::vault::tests::test_encrypt_decrypt' panicked at src/commands/vault.rs:1350:57:
       > called `Result::unwrap()` on an `Err` value: ExcessiveWork { required: 18, target: 13 }
       > note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
       >
       >
       > failures:
       >     commands::vault::tests::test_encrypt_decrypt
       >
       > test result: FAILED. 70 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 8.87s
       >