vfmt: keep file permissions with -w on !windows (#13334)

vlib/os/inode_test.v

@@ -41,3 +41,16 @@ fn test_inode_file_owner_permission() {
 	assert mode.owner.write
 	assert !mode.owner.execute
 }
+
+fn test_inode_file_permissions_bitmask() {
+	if user_os() == 'windows' {
+		println('> skipping ${@FN} on windows')
+		return
+	}
+	filename := './test3.txt'
+	mut file := open_file(filename, 'w', 0o641) or { return }
+	file.close()
+	mode := inode(filename)
+	rm(filename) or {}
+	assert mode.bitmask() == 0o641
+}