fix cargo install twoliter

Unfortunately the cargo install command does not seem to respect the
rust-toolchain.toml and .cargo/config.toml settings when installing from
a git reference.

rust-lang/cargo#11036

We need to fix this by adding +nightly and specifying -Z bindeps in the
cargo install command for twoliter.

tools/install-twoliter.sh

@@ -146,7 +146,8 @@ else
 fi
 
 if [ "${from_source}" = "true" ] ; then
-   cargo install \
+   cargo +nightly install \
+     -Z bindeps \
      --locked \
      --root "${workdir}" \
      --git "${repo}" \