Replace the misleading OR (`||`) with pipe (`|`) in gitlab-ci.yml documentation

What does this MR do?

There is a bug in the demo code snippet of scripting a gitlab-ci job. The OR (||) operator was used when getting the exit code of a previous command:

job:
  script:
    - false || exit_code=$?
    - if [ $exit_code -ne 0 ]; then echo "Previous command failed"; fi;

As the two commands are connected with the OR (||) operator - if the first command is successfully executed, the second command exit_code=$? will not run. In that case, $exit_code will be empty rather than the expected 0, leading to a false alarm in the following if block.

Suggest to replace the OR operator (||) with a pipe (|) operator that ensures the second command exit_code=$? always being executed:

job:
  script:
    - false | exit_code=$?
    - if [ $exit_code -ne 0 ]; then echo "Previous command failed"; fi;

Related issues

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