[ci] update SWIG installation on macOS (#3477)

* Update .vsts-ci.yml

* Update setup.sh

* Update .vsts-ci.yml

* Update .vsts-ci.yml

* try not to hardcode path

.ci/setup.sh

@@ -15,7 +15,7 @@ if [[ $OS_NAME == "macos" ]]; then
         brew install open-mpi
     fi
     if [[ $AZURE == "true" ]] && [[ $TASK == "sdist" ]]; then
-        brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/f3544543a3115023fc7ca962c21d14b443f419d0/Formula/swig.rb  # swig 3.0.12
+        brew install swig@3
     fi
     wget -q -O conda.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-MacOSX-x86_64.sh
 else  # Linux

.vsts-ci.yml

@@ -91,6 +91,7 @@ jobs:
       CONDA=$AGENT_HOMEDIRECTORY/miniconda
       echo "##vso[task.setvariable variable=CONDA]$CONDA"
       echo "##vso[task.prependpath]$CONDA/bin"
+      echo "##vso[task.prependpath]$(brew --prefix swig@3)/bin"
       echo "##vso[task.setvariable variable=JAVA_HOME]$JAVA_HOME_8_X64"
     displayName: 'Set variables'
   - bash: $(Build.SourcesDirectory)/.ci/setup.sh