CI: re-add perl test module install

Author: davidjb

.github/workflows/build.yml

@@ -39,7 +39,7 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install prerequisites
         run: |
-          sudo apt install -y libpcre3-dev libssl-dev zlib1g-dev cpanminus
+          sudo apt install -y libpcre3-dev libssl-dev zlib1g-dev
       - name: Build
         env:
           _NGINX_VERSION: ${{ matrix.nginx }}
@@ -58,6 +58,8 @@ jobs:
             echo "SHIB_MODULE_PATH=$(pwd)/objs" >> $GITHUB_ENV
       - name: Test
         run: |
+          sudo apt install -y cpanminus
+          cpanm --notest --local-lib=$HOME/perl5 Test::Nginx
           PERL5LIB=$HOME/perl5/lib/perl5 TEST_NGINX_VERBOSE=true prove -v
       - name: Output debugging info on failure
         if: ${{ failure() }}