ci: debug github push

- Adding github.com as known host (https://stackoverflow.com/a/29380672)

Refs #11
2 jobs for master in 2 minutes and 30 seconds (queued for 12 seconds)
Status Name Job ID Coverage
  Check
failed check #202203
docker

00:02:30

 
  Github
skipped github #202204
docker
 
Name Stage Failure
failed
check Check

[ FAIL 1 | WARN 0 | SKIP 2 | PASS 1 ]
Error: Test failures
Execution halted

1 error ✖ | 0 warnings ✔ | 2 notes ✖
Error: R CMD check found ERRORs
Execution halted
Cleaning up file based variables
ERROR: Job failed: exit code 1