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, 1 insertions, 1 deletions

diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index 8c0f20475..56c014a8c 100644
--- a/src/libfetchers/tarball.cc
+++ b/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},
     });