Publishing nightly build on fdroid.

[MohitMaliFtechiz]

Fixes #1777

• Publishing the fdroid nightly on our server location = https://download.kiwix.org/fdroid/repo.
• Updated our fdroid_nightly workflow to install the fdroidserver, and create the metadata and other stuff. Then this workflow publishes the APK and metadata on our server.
• This workflow will trigger once a week(on every Monday), or we trigger this via pushing fdroid_nightly tag.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.66%. Comparing base (02de247) to head (8cf87b4).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4437      +/-   ##
============================================
+ Coverage     58.47%   58.66%   +0.18%     
- Complexity     1461     1468       +7     
============================================
  Files           327      327              
  Lines         17045    17045              
  Branches       2109     2109              
============================================
+ Hits           9967     9999      +32     
+ Misses         5683     5634      -49     
- Partials       1395     1412      +17     

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[MohitMaliFtechiz]

@kelson42 Now CI is correctly working. fdroid update --create-metadata is correctly generating the index-v1.jar and index.xml and other stuff https://github.com/kiwix/kiwix-android/actions/runs/18279918607/job/52040385529?pr=4437#step:7:80. The only error in the CI is it does not have permission to upload it to download.kiwix.org/fdroid See https://github.com/kiwix/kiwix-android/actions/runs/18279918607/job/52040385529?pr=4437#step:8:71.