[ci] bump gcc version for macOS (#3208)

* update gcc version

* Update setup.sh

* fix GitHub Actions

.ci/setup.sh

@@ -9,6 +9,12 @@ if [[ $OS_NAME == "macos" ]]; then
     else  # gcc
         if [[ $TASK != "mpi" ]]; then
             brew install gcc
+            if [[ $GITHUB_ACTIONS == "true" ]]; then
+                brew update
+            fi
+            if [[ $TRAVIS == "true" ]] || [[ $GITHUB_ACTIONS == "true" ]]; then
+                brew upgrade gcc
+            fi
         fi
     fi
     if [[ $TASK == "mpi" ]]; then

.ci/test.sh

 #!/bin/bash
 
 if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "gcc" ]]; then
-    export CXX=g++-9
-    export CC=gcc-9
+    export CXX=g++-10
+    export CC=gcc-10
 elif [[ $OS_NAME == "linux" ]] && [[ $COMPILER == "clang" ]]; then
     export CXX=clang++
     export CC=clang