Reconsider clearing `MAKEFLAGS` when `!buildflags` is used
# Description:
As of https://gitlab.archlinux.org/pacman/pacman/-/commit/09e82f01ea824f9203b0ed7238d98b612669b1a9 using `options=(!buildflags)` also clears `MAKEFLAGS`. I am not sure whether this change was intentional. Before `MAKEFLAGS` where preserved and `!buildflags` only cleared other variables such as `CFLAGS`. The `!buildflags` option is very useful for cross compilation (e.g. for the many `mingw-w64-*` and `android-*` packages found in the AUR). However, the previous behavior of still preserving `MAKEFLAGS` made much more sense because flags like `-j$(nproc)` can and should still apply for those cross-packages.
So it would be nice to reconsider this change and preserve `MAKEFLAGS` again (despite `!buildflags`). It looks like this change was not intentional anyway because the mentioned change is focusing on Rust.
# Additional info:
In case anybody else wants to build those cross-packages with custom `MAKEFLAGS`, here's my current workaround: https://github.com/Martchus/PKGBUILDs/tree/master/buildenv/default
I personally was fine using the mentioned workaround but considering this affects probably many people's builds (e.g. see [this discussion on GitHub](https://github.com/Martchus/PKGBUILDs/discussions/145#discussioncomment-8987924)) it would probably be better if pacman would do the right thing for this use case again out of the box.
issue