PPA: Fix libzim-dev dependency

[legoktm]

Our libzim packages are "7.2.0~focal" but the ~ means that "7.2.0" is greater than
"7.2.0~focal" so the dependency can't be satisfied. Depending on "7.2.0~" will
allow "7.2.0~focal" to satisfy the dependency.

[codecov]

Codecov Report

Merging #284 (724fc10) into master (09fad0d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #284   +/-   ##
=======================================
  Coverage   51.13%   51.13%           
=======================================
  Files          21       21           
  Lines        1936     1936           
  Branches     1146     1147    +1     
=======================================
  Hits          990      990           
  Misses        944      944           
  Partials        2        2           

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