d/rules: close race between 'cp' and module handling

d/rules: close race between 'cp' and module handling

sometimes the build would fail with

cp: cannot stat 'ubuntu-hirsute/.tmp_1987275': No such file or directory
make[1]: *** [debian/rules:181: .headers_prepare_mark] Error 1
make[1]: Leaving directory '/home/fgruenbichler/pve-kernel/build'
dpkg-buildpackage: error: fakeroot debian/rules binary subprocess returned exit status 2
make: *** [Makefile:58: pve-kernel-5.11.21-1-pve_5.11.21-1_amd64.deb] Error 2

if copying was slow enough.

so let's do the copying first, then do the rest in parallel without
needing to worry about side-effects.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>