Display valid installable in InstallableDerivedPath::parse warning

The warning message should produce an installable name that can be
passed to `nix build`, `nix path-info`, etc. again. Since the CLI
expects that the .drv path and the output names are separated by
a caret, the warning message must also separate the .drv path and output
names with a caret.

However, `DerivedPath::Built.to_string()` uses an exclamation point as
the separator instead. This commit adds a `separator` argument to the
to_string method.

This changes the warning message from:
If this command is now failing try again with '/nix/store/foo.drv!*'
to:
If this command is now failing try again with '/nix/store/foo.drv^*'

src/libstore/derived-path.cc

@@ -59,18 +59,19 @@ std::string DerivedPath::Opaque::to_string(const Store & store) const
     return store.printStorePath(path);
 }
 
-std::string DerivedPath::Built::to_string(const Store & store) const
+std::string DerivedPath::Built::to_string(const Store & store, char separator) const
 {
     return store.printStorePath(drvPath)
-        + "!"
+        + separator
         + outputs.to_string();
 }
 
-std::string DerivedPath::to_string(const Store & store) const
+std::string DerivedPath::to_string(const Store & store, char separator) const
 {
-    return std::visit(
-        [&](const auto & req) { return req.to_string(store); },
-        this->raw());
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & req) { return req.to_string(store, separator); },
+        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+    }, this->raw());
 }