Update .gitlab-ci.yml

fix nightly test
- rustup default nightly

.gitlab-ci.yml

@@ -618,7 +618,7 @@ test-rust-nightly:
     - git submodule update --init --recursive
     - export RUST_FILES_MODIFIED=$(git --no-pager diff --name-only $CI_BUILD_REF^ $CI_BUILD_REF | grep -v -e ^js -e ^\\. -e ^LICENSE -e ^README.md -e ^appveyor.yml -e ^test.sh -e ^windows/ -e ^scripts/ -e^mac/ -e ^nsis/ | wc -l)
   script:
-    - rustup default stable
+    - rustup default nightly
     - export RUST_BACKTRACE=1
     - if [ $RUST_FILES_MODIFIED -eq 0 ]; then echo "Skipping Rust tests since no Rust files modified."; else ./test.sh $CARGOFLAGS; fi
   tags: