REGR: setting column with setitem should not modify existing array inplace
Closes #33457

This changes `ExtensionBlock.setitem` to how it was before #32831: updating the Block itself inplace, but not the array. 

@jbrockmendel I know you won't like that this update the Block inplace (it's also not exactly what you suggested at https://github.com/pandas-dev/pandas/issues/33457#issuecomment-654367183). But this was the smallest change I could find to fix the issue. 

The alternative is probably changing `BlockManager.iset` to not use Block.set in case of an existing ExtensionBlock, but `BlockManager.iset` is quite complex ..
