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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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