cc-wrapper: ensure that $PATH is exported

After #400734 we modify gcc's $PATH in add-flags.sh which means that
we now need to export it if it was not set originally, otherwise gcc's
environment will not include the modification.

pkgs/build-support/cc-wrapper/cc-wrapper.sh

@@ -249,7 +249,7 @@ if (( "${NIX_DEBUG:-0}" >= 1 )); then
     printf "  %q\n" ${extraAfter+"${extraAfter[@]}"} >&2
 fi
 
-PATH="$path_backup"
+export PATH="$path_backup"
 # Old bash workaround, see above.
 
 if (( "${NIX_CC_USE_RESPONSE_FILE:-@use_response_file_by_default@}" >= 1 )); then

pkgs/build-support/cc-wrapper/gnat-wrapper.sh

@@ -174,7 +174,7 @@ if (( "${NIX_DEBUG:-0}" >= 1 )); then
     printf "  %q\n" ${extraAfter+"${extraAfter[@]}"} >&2
 fi
 
-PATH="$path_backup"
+export PATH="$path_backup"
 # Old bash workaround, see above.
 exec @prog@ \
     ${extraBefore+"${extraBefore[@]}"} \