only install recommended modules on new enough Perls

We need to do this because IO::Compress::Brotli requires Perl 5.14, but
we don't want to depend on that. So we've made it a recommended
dependency, which we still want to install to run tests with on all
Perls that allow it. But we need to explicitly not attempt to install it
on Perls that are too old, because cpm cannot determine that a module's
dependencies aren't met and ignore it.

Author: simbabque

.github/workflows/build-and-test.yml

@@ -58,6 +58,10 @@ jobs:
       env:
         AUTHOR_TESTING: 1
     steps:
+      - name: Decide if we need to install recommended modules
+        id: with-recommends
+        if: matrix.perl-version >= 5.14
+        run: echo '::set-output name=flag::--with-recommends'
       - uses: actions/download-artifact@master
         with:
           name: build_dir
@@ -68,7 +72,7 @@ jobs:
           cpm install -g
           --cpanfile cpanfile
           --with-develop
-          --with-recommends
+          ${{ steps.with-recommends.outputs.flag }}
           --with-suggests
           --show-build-log-on-failure
       - name: Run Tests
@@ -103,6 +107,10 @@ jobs:
         uses: shogo82148/actions-setup-perl@v1
         with:
           perl-version: ${{ matrix.perl-version }}
+      - name: Decide if we need to install recommended modules
+        id: with-recommends
+        if: matrix.perl-version >= 5.14
+        run: echo '::set-output name=flag::--with-recommends'
       - uses: actions/download-artifact@master
         with:
           name: build_dir
@@ -112,7 +120,13 @@ jobs:
         uses: perl-actions/install-with-cpm@v1
         with:
           cpanfile: "cpanfile"
-          args: "--with-develop --with-recommends --with-suggests --with-test --mirror https://cpan.metacpan.org --mirror http://cpan.cpantesters.org"
+          args: > 
+            --with-develop
+            ${{ steps.with-recommends.outputs.flag }}
+            --with-suggests
+            --with-test
+            --mirror https://cpan.metacpan.org
+            --mirror http://cpan.cpantesters.org
       - run: prove -l t xt
         env:
           AUTHOR_TESTING: 1