[ci] replace uses of backticks in test.sh with `$()` (#4519)

.ci/test.sh

@@ -40,9 +40,9 @@ if [[ $TASK == "check-docs" ]] || [[ $TASK == "check-links" ]]; then
     pip install --user -r requirements.txt
     # check reStructuredText formatting
     cd $BUILD_DIRECTORY/python-package
-    rstcheck --report warning `find . -type f -name "*.rst"` || exit -1
+    rstcheck --report warning $(find . -type f -name "*.rst") || exit -1
     cd $BUILD_DIRECTORY/docs
-    rstcheck --report warning --ignore-directives=autoclass,autofunction,doxygenfile `find . -type f -name "*.rst"` || exit -1
+    rstcheck --report warning --ignore-directives=autoclass,autofunction,doxygenfile $(find . -type f -name "*.rst") || exit -1
     # build docs
     make html || exit -1
     if [[ $TASK == "check-links" ]]; then