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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 92ff224f7..db1ced5d6 100644
--- a/src/libfetchers/github.cc
+++ b/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);