gdb: Fix abort w/NIX_DEBUG_INFO_DIRS patch (#80929)

The current gdb patch to support NIX_DEBUG_INFO_DIRS fails if
the user attempts to change the debug file directory to a
value other than the default. For instance:

(gdb) set debug-file-directory /run/booted-system/sw/lib/debug

results in:

munmap_chunk(): invalid pointer
Aborted (core dumped)

To fix this issue, the debug_file_directory is allocated
with xstrdup so that a subsequent call to xfree will succeed.

pkgs/development/tools/misc/gdb/debug-info-from-env.patch

@@ -3,11 +3,13 @@ of directories with separate debugging information files.
 
 --- a/gdb/main.c
 +++ b/gdb/main.c
-@@ -551,3 +551,6 @@ captured_main_1 (struct captured_main_args *context)
+@@ -556,3 +556,8 @@ captured_main_1 (struct captured_main_args *context)
  
 -  debug_file_directory = relocate_gdb_directory (DEBUGDIR,
 +  debug_file_directory = getenv("NIX_DEBUG_INFO_DIRS");
 +
-+  if (debug_file_directory == NULL)
++  if (debug_file_directory != NULL)
++    debug_file_directory = xstrdup(debug_file_directory);
++  else
 +    debug_file_directory = relocate_gdb_directory (DEBUGDIR,
  						 DEBUGDIR_RELOCATABLE);