cabal-install: Use NubList for list fields of SavedConfig.
User config fields like 'remote-repo', 'extra-prog-path' and 'build-summary' were lists, which mean that when two SavedConfig types were concatentated the lists for these fields were concatenated, resulting in duplicate entries. Duplicated entries in these fields simply don't make sense, so we swicth to NubList for these fields. Closes: #2160
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