Rewrite versionFromFile using maybe

To avoid this warning:

    main/Main.hs:54:3: warning: [-Woverlapping-patterns]
        Pattern match is redundant
        In a case alternative: Just ver -> ...
       |
    54 |   Just ver -> unpack ver
       |   ^^^^^^^^^^^^^^^^^^^^^^

Which happens because to the compiler, the Template Haskell expression
looks like a constant, always making one pattern redundant.

It won't dig into that if we use a function on it though :)

main/Main.hs

@@ -50,9 +50,7 @@ data Nixfmt = Nixfmt
   deriving (Show, Data, Typeable)
 
 versionFromFile :: String
-versionFromFile = case $(embedFileIfExists ".version") of
-  Just ver -> unpack ver
-  _ -> showVersion version
+versionFromFile = maybe (showVersion version) unpack $(embedFileIfExists ".version")
 
 options :: Nixfmt
 options =