omnibus: python: probe the build environment for CC/CXX

The current running environment doesn't have these variables set on
purpose, so we need to check inside the one that will be used for the
build commands.
This fixes a regression introduced in #32230

omnibus/config/software/python3.rb

@@ -54,11 +54,11 @@
 
     # Don't forward CC and CXX to python extensions Makefile, it's quite unlikely that any non default
     # compiler we use would end up being available in the system/docker image used by customers
-    if linux_target? && ENV["CC"]
+    if linux_target? && env["CC"]
       command "sed -i \"s/^CC=[[:space:]]*${CC}/CC=gcc/\" #{install_dir}/embedded/lib/python#{major}.#{minor}/config-3.12-*-linux-gnu/Makefile", :env => env
       command "sed -i \"s/${CC}/gcc/g\" #{install_dir}/embedded/lib/python#{major}.#{minor}/_sysconfigdata__linux_*-linux-gnu.py", :env => env
     end
-    if linux_target? && ENV["CXX"]
+    if linux_target? && env["CXX"]
       command "sed -i \"s/^CXX=[[:space:]]*${CXX}/CC=g++/\" #{install_dir}/embedded/lib/python#{major}.#{minor}/config-3.12-*-linux-gnu/Makefile", :env => env
       command "sed -i \"s/${CXX}/g++/g\" #{install_dir}/embedded/lib/python#{major}.#{minor}/_sysconfigdata__linux_*-linux-gnu.py", :env => env
     end