post_version.py: update the files in the current worktree, not the one with the script that we run

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5928>
diff --git a/bin/post_version.py b/bin/post_version.py
index febe531..328a48b 100755
--- a/bin/post_version.py
+++ b/bin/post_version.py
@@ -27,7 +27,7 @@
 
 
 def update_release_notes(version: str) -> None:
-    p = pathlib.Path(__file__).parent.parent / 'docs' / 'relnotes.rst'
+    p = pathlib.Path('docs') / 'relnotes.rst'
 
     with open(p, 'r') as f:
         relnotes = f.readlines()
@@ -48,7 +48,7 @@
 
 
 def update_calendar(version: str) -> None:
-    p = pathlib.Path(__file__).parent.parent / 'docs' / 'release-calendar.rst'
+    p = pathlib.Path('docs') / 'release-calendar.rst'
 
     with open(p, 'r') as f:
         calendar = f.readlines()