Fix build failure

[mikenye]

Deal with this: https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:622

[mikenye]

#15 46.57 + curl --output - https://opensky-network.org/files/firmware/opensky.gpg.pub
[603](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:605)
#15 46.80   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
[604](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:606)
#15 46.80                                  Dload  Upload   Total   Spent    Left  Speed
[605](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:607)
#15 46.80 
[606](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:608)
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[607](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:609)
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[608](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:610)
#15 47.31 curl: (35) OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to opensky-network.org:443 
[609](https://github.com/sdr-enthusiasts/docker-opensky-network/runs/6647372353?check_suite_focus=true#step:11:611)