only install GNU sed on Macs

Author: arik-so

.github/actions/install-dependencies/action.yml

@@ -14,6 +14,7 @@ runs:
     steps:
         -   name: Install Homebrew Dependencies
             shell: bash
+            if: ${{ runner.os == 'macOS' }}
             run: |
                 brew install gnu-sed
         -   name: Checkout Rust-Lightning and LDK-C-Bindings git