std.fs.Dir.statFile: use fstatat

This avoids extra syscalls.

src/Module.zig

@@ -4137,14 +4137,19 @@ pub fn populateBuiltinFile(mod: *Module) !void {
             };
         }
     } else |err| switch (err) {
-        error.BadPathName => unreachable, // it's always "builtin.zig"
         error.NameTooLong => unreachable, // it's always "builtin.zig"
-        error.PipeBusy => unreachable, // it's not a pipe
-        error.WouldBlock => unreachable, // not asking for non-blocking I/O
-
         error.FileNotFound => try writeBuiltinFile(file, builtin_pkg),
-
-        else => |e| return e,
+        else => |e| {
+            if (builtin.os.tag == .windows) {
+                switch (e) {
+                    error.BadPathName => unreachable, // it's always "builtin.zig"
+                    error.PipeBusy => unreachable, // it's not a pipe
+                    error.WouldBlock => unreachable, // not asking for non-blocking I/O
+                    else => return e,
+                }
+            }
+            return e;
+        },
     }
 
     file.tree = try std.zig.parse(gpa, file.source);