rust: we need the build system snapshot, not host

We need the Rust snapshot (stage0) for the build environment not the
host environment. This is not affecting us currently because we are only
building native versions of this today but should improve the behavior
of meta-rust#81 or if any Rust components are built for the target machine.

recipes-devtools/rust/rust-source-1.10.0.inc

@@ -6,7 +6,7 @@
 RS_VERSION = "1.9.0"
 RS_KEY = "e8edd0fd"
 
-RUST_SNAPSHOT = "rustc-${RS_VERSION}-${RUST_HOST_SYS}"
+RUST_SNAPSHOT = "rustc-${RS_VERSION}-${RUST_BUILD_SYS}"
 
 # These are x86_64-unknown-linux-gnu hashes, how can we add more?
 SRC_URI[rust-snapshot.md5sum] = "f1cf6d2fe15e4be18a08259f1540a4ae"