CI: Install libcurl4-openssl-dev in ubuntu-latest runner

Turns out the `curl-config` failure is related to these:
- actions/runner-images#7958
- actions/runner-images#7945

Explicitly installing curl on macOS shouldn't be necessary, while on `ubuntu-latest`, we have to install `libcurl4-openssl-dev`.

.github/workflows/ci.yml

@@ -38,11 +38,11 @@ jobs:
           opam-disable-sandboxing: true
 
       - name: Install system dependencies (Linux)
-        run: sudo apt-get install libev-dev libonig-dev libcurl4-gnutls-dev
+        run: sudo apt-get install libev-dev libonig-dev libcurl4-openssl-dev
         if: runner.os == 'Linux'
 
       - name: Install system dependencies (macOS)
-        run: brew install libev curl openssl@3 # Openssl is a workaround for https://github.com/ocaml/opam-repository/issues/19676
+        run: brew install libev openssl@3 # Openssl is a workaround for https://github.com/ocaml/opam-repository/issues/19676
         if: runner.os == 'macOS'
 
       - name: Install opam dependencies