ci: extend testing for Linux/OS X on Conda

.github/workflows/macos-conda.yml → .github/workflows/macos-linux-conda.yml

-name: Build Pinocchio for Mac OS X via Conda
+name: Build Pinocchio for Mac OS X/Linux via Conda
 
 on: [push,pull_request]
 
 jobs:
-  pinocchio-osx-conda:
-    runs-on: macos-latest
+  pinocchio-conda:
+    name: Pinocchio on ${{ matrix.os }} with Conda
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os: ["ubuntu-latest", "macos-latest"]
 
     steps:
     - uses: actions/checkout@v2