nix: fix build of dependencies

[pinotree]

• fix the cmakeFlags specification for cereal, as it must be a string (and not a simple string)
• build immer and zug explicitly disabling tests and examples, as they are neither run nor installed

[arximboldi]

Nice one! Will merge once the tests run.

[codecov-commenter]

Codecov Report

Merging #187 (3860a30) into master (a2eceee) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 3860a30 differs from pull request most recent head 9547f91. Consider uploading reports for the commit 9547f91 to get more accurate results

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@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
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  Files          81       81           
  Lines        2875     2875           
=======================================
  Hits         2725     2725           
  Misses        150      150