fixes lftp failing to verify sites secured with letsencrypt.
- more specifically, lftp doesn't validate the cross-signed ISRG Root X1 correctly[1][2].
- this issue is not present when built against openssl.
- a fix for the gnutls codepath has been merged[3], but the project has not seen a release since 2020.
- given this, and the questionable quality of gnutls, it seems reasonable to build with openssl instead.
reproducing this bug yields the following:
> Fatal error: Certificate verification: Not trusted (93:3C:6D:DE:E9:5C:9C:41:A4:0F:9F:50:49:3D:82:BE:03:AD:87:BF)
[1]: https://askubuntu.com/questions/1366456/lftp-certificate-suddenly-not-trusted#comment2395548_1366818
[2]: https://github.com/lavv17/lftp/issues/641
[3]: https://github.com/lavv17/lftp/pull/642
Change-Id: Ib161d8741f6d6debde8a65d94a6c1965b23f82ff
0f80873aa4