Add basic CI

[brainstorm]

Description of changes: Add continuous integration via GHA to catch issues like #2 early.

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[brainstorm]

@nmoutschen I guess you noticed already, but I'll be explicit: I'd suggest not merging this until issue #2 is fixed, because it also fails the same way on CI anyway, see: https://github.com/brainstorm/serverless-rust-demo/runs/3762807603#step:9:91

[nmoutschen]

I've added gcc-aarch64-linux-gnu, which fixes the CI.

For issues in #2 and more, I need to continue experimenting on setting up and/or using the right toolchain for cross--compilation.

[bnusunny]

For musl builds, check out https://musl.cc/. Works pretty well.

[brainstorm]

For musl builds, check out https://musl.cc/. Works pretty well.

Check awslabs/aws-lambda-rust-runtime#306... MUSL introduces unnecessary static bloat when provided.al2 already supports native GLIBC binaries, plus you can relatively easily change the allocator to optimize performance further.