Cast variants fully for libc++10

libc++10 seems to be stricter on what it allows in variant conversion.
I'm not sure what the rules are here, but this is the minimal change
needed to get through the compilation errors.

src/libexpr/eval-cache.cc

@@ -394,7 +394,7 @@ Value & AttrCursor::forceValue()
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
         else if (v.type == tPath)
-            cachedValue = {root->db->setString(getKey(), v.path), v.path};
+            cachedValue = {root->db->setString(getKey(), v.path), string_t{v.path, {}}};
         else if (v.type == tBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
         else if (v.type == tAttrs)

src/libexpr/primops/fetchTree.cc

@@ -104,7 +104,7 @@ static void fetchTree(
             else if (attr.value->type == tBool)
                 attrs.emplace(attr.name, Explicit<bool>{attr.value->boolean});
             else if (attr.value->type == tInt)
-                attrs.emplace(attr.name, attr.value->integer);
+                attrs.emplace(attr.name, uint64_t(attr.value->integer));
             else
                 throw TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
                     attr.name, showType(*attr.value));

src/libfetchers/attrs.cc

@@ -11,7 +11,7 @@ Attrs jsonToAttrs(const nlohmann::json & json)
 
     for (auto & i : json.items()) {
         if (i.value().is_number())
-            attrs.emplace(i.key(), i.value().get<int64_t>());
+            attrs.emplace(i.key(), i.value().get<uint64_t>());
         else if (i.value().is_string())
             attrs.emplace(i.key(), i.value().get<std::string>());
         else if (i.value().is_boolean())

src/libfetchers/github.cc

@@ -195,14 +195,14 @@ struct GitArchiveInputScheme : InputScheme
 
         auto [tree, lastModified] = downloadTarball(store, url.url, "source", true, url.headers);
 
-        input.attrs.insert_or_assign("lastModified", lastModified);
+        input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
 
         getCache()->add(
             store,
             immutableAttrs,
             {
                 {"rev", rev->gitRev()},
-                {"lastModified", lastModified}
+                {"lastModified", uint64_t(lastModified)}
             },
             tree.storePath,
             true);

src/libfetchers/mercurial.cc

@@ -301,7 +301,7 @@ struct MercurialInputScheme : InputScheme
 
         Attrs infoAttrs({
             {"rev", input.getRev()->gitRev()},
-            {"revCount", (int64_t) revCount},
+            {"revCount", (uint64_t) revCount},
         });
 
         if (!_input.getRev())

src/libfetchers/tarball.cc

@@ -152,7 +152,7 @@ std::pair<Tree, time_t> downloadTarball(
     }
 
     Attrs infoAttrs({
-        {"lastModified", lastModified},
+        {"lastModified", uint64_t(lastModified)},
         {"etag", res.etag},
     });