dib: unset all temporary dirs envvars in fake-sudo

The real sudo does it as well, and leaving them when preserving the
environment (-E) maybe breaks the applications, as e.g. chroot will have
a TMPDIR path pointing outside of it.

dib/dib.ml

@@ -301,6 +301,11 @@ if [ -z \"$preserve_env\" ]; then
     esac
   done
 fi
+# TMPDIR needs to be unset, regardless of -E
+unset TMPDIR
+# ... and do that also to the other \"TMPDIR\"-like variables
+unset TMP
+unset TMP_DIR
 
 cmd=$1
 shift