devenv: Don't use Path.relative_to() to resolve relative paths

It's utterly broken, and only works when one path is inside the other:

Traceback (most recent call last):
  File "/usr/lib/python3.12/site-packages/mesonbuild/mesonmain.py", line 194, in run
    return options.run_func(options)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/mesonbuild/mdevenv.py", line 188, in run
    write_gdb_script(privatedir, install_data, workdir)
  File "/usr/lib/python3.12/site-packages/mesonbuild/mdevenv.py", line 142, in write_gdb_script
    rel_path = gdbinit_path.relative_to(workdir_path)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/pathlib.py", line 682, in relative_to
    raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")
ValueError: '/path/to/builddir/.gdbinit' is not in the subpath of '/path/to/workdir'

ERROR: Unhandled python exception

    This is a Meson bug and should be reported!

mesonbuild/mdevenv.py

@@ -10,7 +10,7 @@ import typing as T
 from pathlib import Path
 from . import build, minstall
 from .mesonlib import (EnvironmentVariables, MesonException, is_windows, setup_vsenv, OptionKey,
-                       get_wine_shortpath, MachineChoice)
+                       get_wine_shortpath, MachineChoice, relpath)
 from . import mlog
 
 
@@ -140,7 +140,7 @@ def write_gdb_script(privatedir: Path, install_data: 'InstallData', workdir: Pat
         if first_time:
             gdbinit_path = gdbinit_path.resolve()
             workdir_path = workdir.resolve()
-            rel_path = gdbinit_path.relative_to(workdir_path)
+            rel_path = Path(relpath(gdbinit_path, workdir_path))
             mlog.log('Meson detected GDB helpers and added config in', mlog.bold(str(rel_path)))
             mlog.log('To load it automatically you might need to:')
             mlog.log(' - Add', mlog.bold(f'add-auto-load-safe-path {gdbinit_path.parent}'),