bootloader: replace stray MAX_PATH with PATH_MAX

On Windows, `pyi_path_fopen()` erroneously uses `MAX_PATH` (260)
instead of `PATH_MAX` (4096) to convert the filename to wide
characters, which causes `MultiByteToWideChar()` call in
`pyi_win32_utils_from_utf8()` to fail with
```
MultiByteToWideChar: The data area passed to a system call is too
small.
```
when we try to open a file with a long filename (> 260).

Due to lack of error checking, the resulting `wfilename` array ends
up with random content, and the `_wfopen()` call either fails
(in read-only mode) or creates a randomly-named file (in write
mode).

bootloader/src/pyi_path.c

@@ -376,10 +376,10 @@ pyi_path_archivefile(char *archivefile, const char *thisfile)
 FILE*
 pyi_path_fopen(const char* filename, const char* mode)
 {
-    wchar_t wfilename[MAX_PATH];
+    wchar_t wfilename[PATH_MAX];
     wchar_t wmode[10];
 
-    pyi_win32_utils_from_utf8(wfilename, filename, MAX_PATH);
+    pyi_win32_utils_from_utf8(wfilename, filename, PATH_MAX);
     pyi_win32_utils_from_utf8(wmode, mode, 10);
     return _wfopen(wfilename, wmode);
 }

news/5617.bugfix.rst

@@ -0,0 +1,2 @@
+Fix ``onefile`` builds failing to extract files when the full target
+path exceeds 260 characters.