libcnb-cargo: Set default target triple based on host architecture

[edmorley]

By default, if I run cargo libcnb package on my M1 Macbook, I get:

buildpacks-python $ cargo libcnb package
🚚 Preparing package directory...
🖥️ Gathering Cargo configuration (for x86_64-unknown-linux-musl)
For cross-compilation from aarch64 macos to x86_64-unknown-linux-musl,
a C compiler and linker for the target platform must be installed:

To install an x86_64 cross-compiler on macOS:
brew install messense/macos-cross-toolchains/x86_64-unknown-linux-musl

You will also need to install the Rust target:
rustup target add x86_64-unknown-linux-musl

❌ Failed to configure Cargo for cross-compilation

I have to instead run this every time:

$ cargo libcnb package --target aarch64-unknown-linux-musl

Now that we're in an era of multi-arch builder images (both for Heroku and Paketo), I think libcnb's development workflows should default to using the host architecture rather than hardcoding x86_64 here:

libcnb.rs/libcnb-cargo/src/cli.rs

Lines 26 to 27 in 3fa2c6c

	/// Build for the target triple
	#[arg(long, default_value = "x86_64-unknown-linux-musl")]

This will mean that anyone is both developing against a single-arch builder, and also has an ARM machine will need to explicitly pass --target x86_64-unknown-linux-musl - however, we should favour making the multi-arch path the happy path rather than vice versa.