CI: GitHub: Nix: make GHC 8.8 nonstrict (#782)

For quite some time 8.8 strict build fails:
```
src/Nix/Expr/Types.hs:179:10: error: [-Wmissing-methods, -Werror=missing-methods]
    • No explicit implementation for
        ‘liftTyped’
    • In the instance declaration for ‘Lift (Fix NExprF)’
    |
179 | instance Lift (Fix NExprF) where
    |          ^^^^^^^^^^^^^^^^^
```

`nomissing-methods` seems like a bad path in this case and module.

I tried to address the issue and so far not constructed the implementation.

Too bad that the issue creeped-up backwards. Strict builds worked until this one
cropped-in.

So lowering the CI strictness until this gets resolved. Opened #783 #784 for that.

Author: Anton-Latukha

.github/workflows/Optional-Nix-dev-env-main.yml

@@ -55,9 +55,10 @@ env:
 
 
 jobs:
+  #  2020-12-29: NOTE: Make builds strict again!
 
   build10:
-    name: "Default Nixpkgs GHC (8.8), strict build"
+    name: "Default Nixpkgs GHC (8.8)"
     runs-on: ubuntu-latest
     #  2020-08-01: NOTE: Due to Nixpkgs brittleness to not block project development Nixpkgs made optional, see commit message for more info
     continue-on-error: true
@@ -78,7 +79,6 @@ jobs:
     - name: "Determined Nix-build"
       env:
         compiler: "default"
-        buildStrictly: "true"
       run: ./build.sh