Bump nightly rust version on CI to 2020-07-08

Author: Aaron1011

ci/rust-version.sh

@@ -24,7 +24,7 @@ fi
 if [[ -n $RUST_NIGHTLY_VERSION ]]; then
   nightly_version="$RUST_NIGHTLY_VERSION"
 else
-  nightly_version=2020-06-10
+  nightly_version=2020-07-08
 fi