Fix variable never mutated warning in swift-backtrace (#80055)

```
stdlib/public/libexec/swift-backtrace/main.swift:590:15: warning: variable 'now' was never mutated; consider changing to 'let' constant
 588 |           let name = target!.name
 589 |           let pid = target!.pid
 590 |           var now = timespec(tv_sec: 0, tv_nsec: 0)
     |               `- warning: variable 'now' was never mutated; consider changing to 'let' constant
 591 |
 592 |           let ext: String
```

Author: MaxDesiatov

stdlib/public/libexec/swift-backtrace/main.swift

@@ -587,7 +587,7 @@ Generate a backtrace for the parent process.
           // If the output path is a directory, generate a filename
           let name = target!.name
           let pid = target!.pid
-          var now = timespec(tv_sec: 0, tv_nsec: 0)
+          let now = timespec(tv_sec: 0, tv_nsec: 0)
 
           let ext: String
           switch args.format {