ci(system-tests): fix ddtrace py build from source (#690)

* ci(system-tests): fix ddtrace py build from source

* ci(system-tests): run on pull requests

Author: florentinl

.github/workflows/build_layer.yml

@@ -2,6 +2,7 @@ name: Build Layers for system-Tests
 
 on:
   workflow_dispatch:
+  pull_request:
   push:
     branches:
       - "main"

Dockerfile

@@ -15,6 +15,7 @@ RUN set -eux; \
       rm gcc && ln -s gcc10-gcc gcc; \
       rm g++ && ln -s gcc10-g++ g++; \
       rm cc && ln -s gcc10-cc cc; \
+      rm c++ && ln -s gcc10-c++ c++; \
     fi
 
 # Add Rust compiler which is needed to build dd-trace-py from source