os: fix use of deprecated splitBackwards for Flatpak (ghostty-org#6733)

Author: pluiedev

src/os/passwd.zig

@@ -116,7 +116,7 @@ pub fn get(alloc: Allocator) !Entry {
         };
 
         // Shell and home are the last two entries
-        var it = std.mem.splitBackwards(u8, std.mem.trimRight(u8, output, " \r\n"), ":");
+        var it = std.mem.splitBackwardsScalar(u8, std.mem.trimRight(u8, output, " \r\n"), ':');
         result.shell = it.next() orelse null;
         result.home = it.next() orelse null;
         return result;