Fix use of NumJobs

From `Cabal/src/Distribution/Types/ParStrat.hs`

```
data ParStratX sem
  = -- | Compile in parallel with the given number of jobs (`-jN` or `-j`).
    NumJobs (Maybe Int)
  ...
```

However in `Cabal/src/Distribution/Simple/Program/GHC.hs` show is applied to the `Maybe Int` and we get errors like:

```
ghc-9.9.20230901: on the commandline: malformed integer argument in -jJust 4
```

This change should correct the behavior in `Simple/Program/GHC.hs` to match the comment in `Types/ParStrat.hs`.

Cabal/src/Distribution/Simple/Program/GHC.hs

@@ -701,7 +701,7 @@ renderGhcOptions comp _platform@(Platform _arch os) opts
                 if jsemSupported comp
                   then ["-jsem " ++ name]
                   else []
-              Flag (NumJobs n) -> ["-j" ++ show n]
+              Flag (NumJobs n) -> ["-j" ++ maybe "" show n]
             else []
         , --------------------
           -- Creating libraries