chore(ci): unpin cargo lambda in integration test ci

[jlizen]

📬 Issue #, if available:
Closes: #1006

✍️ Description of changes:

Upstream fix has landed: cargo-lambda/cargo-lambda#865

So we should be good to unpin cargo-lambda in our CI. I'll keep an eye to make sure it succeeds after merge (we don't have a convenient way of triggering this workflow on PR).

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.