This is done because the existing fetchCargoTarball does not support pname or version, resulting in a vendor tarball
with the name cargo-deps-vendor.tar.gz. Since adding pname+version support would change the name of the derivation,
and therefore its hash, we remove existing usages to avoid treewide breakage.
This is a temporary fix to get rustc building again with these
configurations (which notably include the default aarch64-darwin one)
without causing a stdenv rebuild. The real fix will be to avoid
passing these options through the Clang wrapper when the target is
overridden.
It could be that there are packages that need wasm32-unknown-unknown
beyond those I've marked as broken here — it's impossible to be sure
without a full rebuild. But this should be most of them.
