No, you don't have ~/.ghc/bin prepended to your $PATH. You have it appended instead, which is your original problem. When the systems looks for the "ghc" binary it will find the older install since it comes first in your path. Try this instead:

export PATH=$HOME/.ghc/bin:$PATH
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and then try again (without exporting the GHC variable, since it should now find the correct version through the PATH variable).