Add integration tests in CI for cross-compile between different architectures/platforms

[edmorley]

We currently have integration tests in CI for packaging a buildpack on an x86_64 Linux host, targetting x86_64-unknown-linux-musl.

However, this isn't a true cross-compile - it's only using musl instead of libc, as the host and the target have the same architecture.

We should test scenarios such as compiling for x86_64 from an ARM64 host.

In addition, we should:

• test all of these against an alpine image, to confirm that musl works (we partly do this in CI at the moment, but the test buildpack doesn't use ring)
• make sure the test includes C based Rust crates such as ring

These should hopefully catch issues like #724 and #726.