fix(publisher-ers): properly publish non-default flavours

[dariusclay]

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• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

• post request form data for version now uses flavor value
• also including auto channel selection for 'rc' versions

[erickzhao]

oops that's my bad, I failed to properly implement the feature in #2766

[erickzhao]

@dariusclay I'm happy to merge this as-is, but I'm wondering if there are any improvements to be made to the tests in accordance to this PR!

[dariusclay]

For sure, I will check the test cases and see what I can do. Thanks