fixup! riotbuild: Install Rust

See-Also: immunant/c2rust#323

Author: chrysn

riotbuild/Dockerfile

@@ -278,7 +278,7 @@ RUN \
     rustup install nightly-2019-12-05 && \
     rustup component add --toolchain nightly-2019-12-05 rustfmt rustc-dev && \
     git clone https://github.com/chrysn-pull-requests/c2rust -b for-riot && \
-    (cd c2rust && cargo +nightly-2019-12-05 install --path c2rust)
+    (cd c2rust && cargo +nightly-2019-12-05 install --path c2rust --locked)
 
 # get Dockerfile version from build args
 ARG RIOTBUILD_VERSION=unknown