[pristine-tar] Update PlatformIO to 6.1.18

This is part of a MR series updating PlatformIO to v6.1.18. This request updates the pristine-tar branch.

Discussion at https://lists.debian.org/debian-python/2025/08/msg00034.html

The pristine-tar branch was updated with this command:

gbp buildpackage --git-upstream-tag='v%(version)s' --git-upstream-tree=BRANCH --git-upstream-branch=upstream --git-debian-branch=debian/master --git-submodules --git-no-pristine-tar --git-pristine-tar-commit

See also:

• !2
• !4