Append CONAN_LIBS and CONAN_SYSTEM_LIBS CMake variables to avoid overwriting user-defined libs

[danimtb]

Changelog: Bugfix: Append CONAN_LIBS in cmake generator to avoid overwriting user-defined libs.
Docs: omit

• Refer to the issue that supports this Pull Request: closes [bug] CONAN_LIBS is overwritten when running conan_basic_setup() #6396
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[jgsogo]

If this is for 1.22, then it should take into account #6298

[danimtb]

Apparently, changes in this PR are not affecting the changes in PR #6298. I have run the tests of #6298 with changes here and it worked.

[jgsogo]

Thanks for taking the time to test these changes taking into account the other PR 🙌 Indeed, that one is about targets and shouldn't interfere.

[memsharded]

Looks good, please create a 1.21.2 release branch and target this PR to it.
Thanks!

[jgsogo]

@memsharded This PR is running Conan v1.21, so Python 3.8 is not supported yet and CI won't pass. This is going to happen in this PR, in branch release/1.21.2 and also in master before merging 1.22.

Right now the CI uses py38 unconditionally, we could add some kind of if/else conditions in the CI source based on the branch name to remove it from the CI matrix, but I feel like it would be cleaner to backport Python 3.8 changes to this branch too (in fact we should backport it to any Conan version we consider alive). 👍 or 👎 ?

[jgsogo]

@danimtb, We are adding Python 3.8 to release/1.21.2, this PR should pass after those changes are available