src: add encoding_type variable in WritePrivateKey

This commit adds a local variable named encoding_type which is set to
the value of the Maybe using ToChecked().

The motivation for this is the code for ToChecked() could be executed
multiple times depending on path taken at runtime. I also think this
improves readability, or at least it is as readable as before this
change.

PR-URL: https://github.com/nodejs/node/pull/34181


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>