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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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