try using less memory during build

```
--  While building package Cabal-2.4.1.0 using:
      /root/.stack/setup-exe-cache/x86_64-linux-nix/Cabal-simple_mPHDZzAJ_2.4.0.1_ghc-8.6.5 --builddir=.stack-work/dist/x86_64-linux-nix/Cabal-2.4.0.1 build --ghc-options "-hpcdir .stack-work/dist/x86_64-linux-nix/Cabal-2.4.0.1/hpc -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure (-9) (THIS MAY INDICATE OUT OF MEMORY)
    Logs have been written to: /root/project/.stack-work/logs/Cabal-2.4.1.0.log
```

.circleci/config.yml

@@ -62,7 +62,7 @@ jobs:
       - run:
           name: "Run Tests"
           command: |
-            nix-shell shell.nix --run "stack test --no-terminal --coverage"
+            nix-shell shell.nix --run "stack build --no-terminal --test --coverage --fast"
 
       - save_cache:
           name: "Cache Dependencies"