fix: ensure that Meilisearch secret is used in workflows

[jorgepiloto]

Continuation of #1302, fixes #1327 by adding the variable at step level. Everything is properly configured in the conf.py file and I can see the Meilisearch indices for PyDPF working as expected.

The latest stable documentation does not contain any reference to Meilisearch in its HTML, meaning that the MEILISEARCH_PUBLIC_API_KEY was not available when the docs built. I suspect that the cmd is starting a new shell with a new env, loosing track of the environment declared in the docs.yml.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9966263) 86.52% compared to head (472abd1) 86.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1400      +/-   ##
==========================================
+ Coverage   86.52%   86.91%   +0.38%     
==========================================
  Files          81       81              
  Lines        9513     9506       -7     
==========================================
+ Hits         8231     8262      +31     
+ Misses       1282     1244      -38     

[MaxJPRey]

Thanks @jorgepiloto for fixing it.