update Makefile

1 job for gen_proofmode in 9 minutes and 31 seconds (queued for 8 minutes and 27 seconds)
Name Stage Failure
build-iris.dev Build
Makefile.coq:317: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 3m42.545s
user 10m49.584s
sys 0m26.128s
ERROR: Job failed: exit code 1