Adds curl to the list of system libraries

Fixes macOS build with system libcurl

linux/build-environment/entrypoint.sh

@@ -38,6 +38,7 @@ cmake_options=(
     -Daudacity_use_sbsms=local # We prefer using the latest version of sbsms
     -Daudacity_use_soundtouch=system
     -Daudacity_use_twolame=system
+    -Daudacity_use_curl=system
 )
 
 cmake "${cmake_options[@]}" ../audacity