Fix to-base --type handler to correctly set std::optional flag

Now that we have a separate flag function, also describe why it is
optional.

src/libutil/args.cc

@@ -166,6 +166,18 @@ Args::Flag Args::Flag::mkHashTypeFlag(std::string && longName, HashType * ht)
     };
 }
 
+Args::Flag Args::Flag::mkHashTypeOptFlag(std::string && longName, std::optional<HashType> * oht)
+{
+    return Flag {
+        .longName = std::move(longName),
+        .description = "hash algorithm ('md5', 'sha1', 'sha256', or 'sha512'). Optional as can also be gotten from SRI hash itself.",
+        .labels = {"hash-algo"},
+        .handler = {[oht](std::string s) {
+            *oht = std::optional<HashType> { parseHashType(s) };
+        }}
+    };
+}
+
 Strings argvToStrings(int argc, char * * argv)
 {
     Strings args;

src/libutil/args.hh

@@ -85,6 +85,7 @@ protected:
         Handler handler;
 
         static Flag mkHashTypeFlag(std::string && longName, HashType * ht);
+        static Flag mkHashTypeOptFlag(std::string && longName, std::optional<HashType> * oht);
     };
 
     std::map<std::string, Flag::ptr> longFlags;

src/nix/hash.cc

@@ -84,7 +84,7 @@ struct CmdToBase : Command
 
     CmdToBase(Base base) : base(base)
     {
-        addFlag(Flag::mkHashTypeFlag("type", &*ht));
+        addFlag(Flag::mkHashTypeOptFlag("type", &ht));
         expectArgs("strings", &args);
     }