Using stable for cargo fmt in GitHub actions

[nmoutschen]

Hey folks! 👋

I've noticed this uses the nightly version of Rust to do fmt and clippy checks (see https://github.com/awslabs/aws-lambda-rust-runtime/blob/master/.github/workflows/build.yml#L60-L81).

Since those versions might include new checks that are not yet in stable, and most people are using stable, should we change to using stable there too?

[coltonweaver]

I think we should 100% use stable. Good call out.

[nmoutschen]

Actually, looking at the bigger picture, we should probably define an MSRV at some point too.

I'll do some tests to see what we could realistically support. In the crate's current stage, we could lower MSRV to 1.46.0 with three small changes:

• Downgrade edition for lambda-extension and lambda-runtime-api-client to 2018; and
• Import std::convert::TryInto for lambda-runtime-api-client.

We can't go any lower that this due to while and if usage in const fn in http-0.2.5.

Counter argument to this is that aligning with the Rust SDK will probably be good enough. We'd still need to make those changes to support 1.53.0 too.

I can make those changes tomorrow and submit a PR.

[bahildebrand]

@nmoutschen can we close this? I see a related PR is merged, but not sure if there is any work remaining.

[nmoutschen]

Yes we can! :)