build-remote: fix format string shenanigans

HintFmt(string) invokes the HintFmt("%s", literal) constructor,
which is not what we want here. Add a constructor with a proper name
and call that.

Next step: rename all the other ones to HintFmt::literal(string).

Fixes https://github.com/NixOS/nix/issues/10238

src/libutil/fmt.hh

@@ -144,6 +144,10 @@ public:
         : HintFmt("%s", Uncolored(literal))
     { }
 
+    static HintFmt fromFormatString(const std::string & format) {
+        return HintFmt(boost::format(format));
+    }
+
     /**
      * Interpolate the given arguments into the format string.
      */