Downgrade rust nightly if necessary to run rustfmt.

Author: ltratt

.buildbot.sh

@@ -6,11 +6,13 @@ export CARGO_HOME="`pwd`/.cargo"
 export RUSTUP_HOME="`pwd`/.rustup"
 
 curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs > rustup.sh
-# Install no toolchain initially to ensure toolchain rollback.
-sh rustup.sh --default-host x86_64-unknown-linux-gnu --default-toolchain none -y --no-modify-path
-
+sh rustup.sh --default-host x86_64-unknown-linux-gnu \
+    --default-toolchain nightly \
+    --no-modify-path \
+    --profile minimal \
+    -y
 export PATH=`pwd`/.cargo/bin/:$PATH
-rustup install nightly
+cargo check
 
+rustup toolchain install nightly --allow-downgrade --component rustfmt
 cargo +nightly fmt --all -- --check
-cargo check