re-enable make validate on CI

2 jobs for master in 10 minutes and 24 seconds (queued for 18 minutes and 40 seconds)
Status Job ID Name Coverage
  Test
failed #1819
coq
iris-coq8.5.3

00:05:16

passed #1820
coq
iris-coq8.6beta

00:05:08

 
Name Stage Failure
failed
iris-coq8.5.3 Test There is an unknown failure, please try again
theories/tests/joining_existentials (user: 9.25 mem: 544312 ko)
make[1]: Leaving directory '/builds/FP/iris-coq'

real 4m10.571s
user 12m11.040s
sys 0m18.403s
$ if fgrep Axiom build-log-full.txt >/dev/null then; exit 1; fi
/usr/bin/bash: eval: line 46: syntax error near unexpected token `fi'
ERROR: Build failed: exit code 2