Support native installers

[hoffa]

Currently setup-sam uses pip to install SAM CLI. It makes multi-platform support trivial, but means issues can occur due to transient dependencies changing (e.g. #64 or #41). It also means Python has to be installed for setup-sam to work (although it isn't typically a problem in practice; GitHub-hosted runners include it).

Using the official installers would minimize dependency issues and get rid of the setup-python dependency. And perhaps makes #12 easier? One potential issue is SAM CLI doesn't currently support Linux ARM (see aws/aws-sam-cli#3908).

No need to run the install script, and can use dist/sam directly instead. Might help with caching.

Should probably push a v3, as the python input will be removed, and don't want to break changes such as #64 (comment).

[hoffa]

Done in #70 for Linux x86-64; which is also ubuntu-latest so assuming the most common. Resolving in favor of more granular ones.

[aidansteele]

Sorry for the spam, but I just wanted to say thank you so much @hoffa. This change has halved the runtime of my entire pipeline for the (fairly common) case of no changes needing deployment. Even when there are changes to deploy, it's going to make a substantial difference - and make our devs happier with submitting PRs. ❤️

[hoffa]

@aidansteele Thanks for you comment, that's great to hear!