Fix a faulty sed command

.github/Dockerfile

@@ -18,7 +18,7 @@ RUN rm -rf local/share/doc local/share/*/doc local/share/*/examples local/share/
 # Remove all __pycache__ directories
 RUN find /sage -type d -name "__pycache__" -exec rm -rf {} +
 # Remove all .c .cpp files corresponding to .pyx files
-RUN find /sage -name "*.pyx" | sed 's/\.pyx$/\.c \0.cpp/' | xargs rm -f +
+RUN find /sage -name "*.pyx" | sed 's/\(.*\)[.]pyx$/\1.c \1.cpp/' | xargs rm -f +
 
 FROM ubuntu:jammy
 # or FROM ghcr.io/sagemath/sage/sage-ubuntu-jammy-minimal-with-system-packages:latest