rust triples are inconsistent across our architectures

The triples we use for rust are inconsistent.

on x86_64 we use x86_64-alpine-linux-musl:

$  apk info -L rust | grep musl | head -n 1
usr/lib/rustlib/x86_64-alpine-linux-musl/lib/crt1.o

On aarch64 we use aarch64-unknown-linux-musl:

$ apk info -L rust | grep musl | head -n 1
usr/lib/rustlib/aarch64-unknown-linux-musl/lib/libtest-1e92eac762a69f5a.so

As you see, all arches uses *-unknown-linux-* while x86_64 uses *-alpine-linux-*.

I think we should be consistent.

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