github CI: prepend to PKG_CONFIG_PATH

.github/workflows/ci.yml

@@ -70,7 +70,7 @@ jobs:
 
     env:
       # Configure pkg-config to use OpenSSL from Homebrew
-      PKG_CONFIG_PATH: /usr/local/opt/openssl@1.1/lib/pkgconfig
+      PKG_CONFIG_PATH: "/usr/local/opt/openssl@1.1/lib/pkgconfig:$PKG_CONFIG_PATH"
       TOXENV: ${{ matrix.toxenv }}
 
     runs-on: ${{ matrix.os }}