Modify GH workflow to always publish to latest

[micrictor]

I ran into some dependency issues recently due to the latest image not reflecting what's in this repo. This change will make it so every commit to this repo generates a new latest image, while still allowing for the automatic creation of specifically tagged images.

I think that this is better all around - less work for the single-threaded owner of this repo, as latest is always up-to-date, consumers of the serverless-rust package automagically get the latest improvements to the build image, and users are still free to select specific versions as needed.

The only thing I'm unsure of is if, given this change, we would then want to set specific versions of the build image for versions of serverless-rust.

[hrishikeshparanjape]

this one should solve https://github.com/softprops/lambda-rust/issues/98 I think.