Fix generation of coverage report

[sevanspowell]

• The coverage reports weren't working because the "doCoverage" haskell.nix option was not enabled, and the option was referring to the wrong package (was "decentralized-software-updates" but the package is called "decentralized-updates").
• Modified the project slightly to more easily toggle generation of coverage information.
• Commented out the "-Werror" configure flag. This flag was not actually activated before because it was referring to the wrong package (see above). After setting it to refer to the correct package, the project no longer builds, so I've commented it out for the time being. Please review whether or not you want this flag.

[sevanspowell]

Buildkite killed coverage step because "Coverage tests" take too long.

I disabled those tests locally and the coverage report ran successfully, so you'll just need to find some way to make the coverage tests run faster, or extend the timeout on Buildkite @dnadales. Coverage report should work from there.