clang-tools: use $C_INCLUDE_PATH for C includes

$CPATH is taken to mean the same thing as -I by clang, whereas
$C_INCLUDE_PATH is -isystem. Using $CPATH means tools like clangd will
generate warnings for problems in system headers, which is undesirable.

See llvm/llvm-project#154015

pkgs/development/compilers/llvm/common/clang-tools/wrapper

@@ -47,8 +47,8 @@ for arg in "$@"; do
 done
 
 if [ "$extendcpath" = true ]; then
-  export CPATH=${CPATH}${CPATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE} \
-                                                 $(<@clang@/nix-support/libc-cflags))
+  export C_INCLUDE_PATH=${CPATH}${CPATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE} \
+                                                          $(<@clang@/nix-support/libc-cflags))
 
   export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}${CPLUS_INCLUDE_PATH:+':'}$(buildcpluspath ${NIX_CFLAGS_COMPILE} \
                                                                                             $(<@clang@/nix-support/libcxx-cxxflags) \