os: faster getFileSize (#19438)

Use "stat" rather than "open", "seek", and "close" system calls.
The Windows implementation remains the same.

lib/pure/os.nim

@@ -3216,11 +3216,10 @@ proc getFileSize*(file: string): BiggestInt {.rtl, extern: "nos$1",
     result = rdFileSize(a)
     findClose(resA)
   else:
-    var f: File
-    if open(f, file):
-      result = getFileSize(f)
-      close(f)
-    else: raiseOSError(osLastError(), file)
+    var rawInfo: Stat
+    if stat(file, rawInfo) < 0'i32:
+      raiseOSError(osLastError(), file)
+    rawInfo.st_size
 
 when defined(windows) or weirdTarget:
   type