Add ftp_proxy and all_proxy proxy variables to fix #5683

[nicememory]

Changelog: Fix: Excluded also ftp_proxy and all_proxy variables from the environment when proxy configuration is specified in the conan.conf file.
Docs: Omit

Fixes #5683

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[jgsogo]

Thanks for your contribution! 🎉

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Fill in the "Changelog" line and, for the docs, you can just use Omit, no docs are needed for this PR.

Thanks

[nicememory]

I've signed the CLA and filled the PR. I'd like to contribute more to conan, it's awesome!