Fix util/find-doc-nits' environment variable check exceptions

Fix util/find-doc-nits' environment variable check exceptions

Some files in @except_env_files are located in the build directory,
not the source directory.

Furthermore, because the files and directories in @except_dirs and
@except_env_files may look different than the elements in what find()
returns, realpath() must be used to ensure that file name comparison
matches when it should.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28601)