Extract logic to pull publishing credentials into a dedicated snippet

[0xnm]

What does this PR do?

Small improvement to the Gitlab CI file: common calls to pull credentials are extracted into a dedicated snippet.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.01%. Comparing base (82871ea) to head (0296277).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2176   +/-   ##
========================================
  Coverage    70.01%   70.01%           
========================================
  Files          726      726           
  Lines        26997    26987   -10     
  Branches      4525     4521    -4     
========================================
- Hits         18900    18893    -7     
  Misses        6829     6829           
+ Partials      1268     1265    -3     

see 31 files with indirect coverage changes