Fail core-lint of `Unpack (Base "m") ~# ('["m"] ':/ '[])`

[philderbeast]

If I add core linting to the test-suite uom-plugin:units then I get errors.

> cabal test uom-plugin:units --test-show-details=always --test-options="--color always"
...
*** Core Lint errors : in result of Desugar (before optimization) ***
<no location info>: warning:
    Non-CoVar has coercion type co_a88o :: Unpack (Base "m")
                                           ~# ('["m"] ':/ '[])
    Substitution: [TCvSubst
                     In scope: InScope {}
                     Type env: []
                     Co env: []]