Use `from_utf8_lossy_owned` for Reading from plugin `stderr` when Stabilized

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(See original comment: https://github.com/icerpc/slicec/pull/776/changes#r3260725518)
(See stabilization tracking issue here)

This is minor, but when we retrieve stderr from a slicec plugin, we get this as a Vec<u8>.
To convert this to a string we have to call:

let mut error_string = String::from_utf8_lossy(&output.stderr).into_owned();

Which is A) verbose, B) makes a copy of all of the bytes.

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There is a nightly proposed function which would be a better fit:

let mut error_string = String::from_utf8_lossy_owned(output.stderr)

This is shorter, and it avoids the copy. Instead of takes ownership of the Vec<u8> and does the conversion in place.