CI: Fix custom API branch install

[greschd]

Fix installing a custom API branch in the CI pipeline:

• build the API repository in a separate step, using Python3.10, since
  newer Python versions are not supported by the necessary protobuf version
• install the API package without dependencies, to avoid accidentally installing
  a transitive dependency which is incompatible with some other dependency

Other changes:

• Simplify the pipeline logic by using env to process the workflow_dispatch inputs.
  This also allows setting a default value in the CI .yml, for example for a feature branch.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.98%. Comparing base (ad8fb06) to head (34ab2bc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #546   +/-   ##
=======================================
  Coverage   91.98%   91.98%           
=======================================
  Files          82       82           
  Lines        4252     4252           
=======================================
  Hits         3911     3911           
  Misses        341      341           

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