https://www.postfix.org/postconf.5.html#import_environment
Be sure to augment rather than replace the default set of variables:
# postconf -e "import_environment = $(postconf -dh import_environment)
SSLKEYLOGFILE=/some/where"
you can also update just the smtpd(8) service in master.cf:
# postconf -e "smtpd_impenv = $(postconf -dh import_environment)
SSLKEYLOGFILE=/some/where"
# postconf -Pe 'smtp/inet/import_environment=$smtpd_impenv'
# postfix reload
Use double and single quotes as indicated.
See the postconf(1) manpage, and if you prefer edit the config files by hand.
Thank yoy !