Use nightly-2020-10-15

Author: JohnTitor

ci/install-rust.sh

@@ -8,7 +8,7 @@ toolchain=
 if [ -n "$TOOLCHAIN" ]; then
   toolchain=$TOOLCHAIN
 else
-  toolchain=nightly
+  toolchain=nightly-2020-10-15
 fi
 if [ "$OS" = "windows" ]; then
   : "${TARGET?The TARGET environment variable must be set.}"