Ensure sparse files on windows during download

The file.Truncate call on windows will write the whole file
unless you set the sparse flag, leading to heavy I/O at the
beginning of download.  This should improve our
I/O behavior on windows and put less stress on the users disk.

server/sparse_common.go

@@ -0,0 +1,9 @@
+//go:build !windows
+
+package server
+
+import "os"
+
+func setSparse(file *os.File) error {
+	return nil
+}