Use user and channel of recipe if not specified

[pgrohs]

Changelog: (Feature): Capture username and channel from conanfile.py

Closes #602

In case user or channel are not given to ConanMultiPackager via parameter or environment variable, both are taken from the recipe to build. The overcomes the necessity to repeat user and channel of the recipe in the build script. Furthermore, it enables the realization of custom channel-strategies in the recipe.

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[uilianries]

@pgrohs Thank you for your contribution. Please, check those failed test. It seems like your feature broke some of them. Probably you need to validate case where user and channel are used, but they are None.

[pgrohs]

@uilianries I have updated the PR. Could please run the test cases?

[uilianries]

@pgrohs Sorry the delay. Starting it now

[pgrohs]

@uilianries Could you please try to run the CI pipeline again?