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: #178

Change-Id: If52d2eb8864ceb8663e05992e9d1fffef573d6b8

src/build-remote/build-remote.cc

@@ -202,7 +202,7 @@ static int main_build_remote(int argc, char * * argv)
                         else
                             drvstr = "<unknown>";
 
-                        auto error = HintFmt(errorText);
+                        auto error = HintFmt::fromFormatString(errorText);
                         error
                             % drvstr
                             % neededSystem

src/libutil/fmt.hh

@@ -141,6 +141,10 @@ 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.
      */