Update .gitlab-ci.yml

1 job for master in 7 seconds
Status Job ID Name Coverage
  Build
failed #32838
build-doc

00:00:07

 
Name Stage Failure
failed
build-doc Build
Created fresh repository.
Checking out 7ea40f24 as master...

Skipping Git submodules setup
Executing "step_script" stage of the job script
$ julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'
/bin/bash: line 97: julia: command not found
Cleaning up file based variables
ERROR: Job failed: exit code 1