Do not use go get to install executables

https://go.dev/doc/go-get-install-deprecation

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

.github/workflows/ci.yml

@@ -88,7 +88,6 @@ jobs:
         shell: bash
         run: |
           echo "GOPATH=${{ github.workspace }}" >> $GITHUB_ENV
-          echo "GO111MODULE=off" >> $GITHUB_ENV
           echo "${{ github.workspace }}/bin" >> $GITHUB_PATH
 
       - name: Install protobuf
@@ -113,7 +112,7 @@ jobs:
         with:
           go-version: '1.17.3'
       - uses: actions/checkout@v2
-      - run: go get github.com/cpuguy83/go-md2man/v2@v2.0.1
+      - run: go install github.com/cpuguy83/go-md2man/v2@v2.0.1
       - run: make man
 
   # Make sure binaries compile with other platforms