starlark-rust: fix rustc version issues (#12404)

Closes: facebook/starlark-rust#128

Author: nathaniel-brough

projects/starlark-rust/Dockerfile

@@ -16,5 +16,6 @@
 
 FROM gcr.io/oss-fuzz-base/base-builder-rust
 RUN git clone --depth 1 https://github.com/facebookexperimental/starlark-rust.git starlark-rust
+RUN rustup update nightly
 WORKDIR starlark-rust
 COPY build.sh $SRC/

projects/starlark-rust/build.sh

@@ -18,7 +18,7 @@
 find . -name '*.star' -exec zip starlark_seed_corpus.zip {} \;
 
 cd starlark
-cargo fuzz build -O
+cargo +nightly fuzz build -O
 
 FUZZ_TARGET_OUTPUT_DIR=fuzz/target/x86_64-unknown-linux-gnu/release
 for f in fuzz/fuzz_targets/*.rs