I was just about to post a very big explanation about how to override certain classes to get this functionality......
And then I found this pull request : https://github.com/laravel/passport/pull/683
So adding this in your env file should work (from PassPort 6 and up):
PASSPORT_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----\nMIIJJwIBAAKCAgEAw3KPag...\n-----END RSA PRIVATE KEY-----"
PASSPORT_PUBLIC_KEY="-----BEGIN PUBLIC KEY-----\nMIICIjANBgkqhkiG9w0BAQEFAAOC...\n-----END PUBLIC KEY-----\n"
// or maybe with escaped \
PASSPORT_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----\nMIIJJwIBAAKCAgEAw3KPag...\n-----END RSA PRIVATE KEY-----"
PASSPORT_PUBLIC_KEY="-----BEGIN PUBLIC KEY-----\nMIICIjANBgkqhkiG9w0BAQEFAAOC...\n-----END PUBLIC KEY-----\n"