generate-ca-certificates: postinstall fails

```
+ mkdir -p /usr/local/ssl/certs
+ ln -s /usr/local/etc/generate-ca-certificates/cert.pem /usr/local/ssl/certs/cert.pem
ln: /usr/local/ssl/certs/cert.pem: File exists
error while installing openssl@1.1.1l: exit status 1
