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>

debian/rules

@@ -114,6 +114,9 @@ binary: install
 .config_mark:
 	cd ${KERNEL_SRC}; scripts/config ${PVE_CONFIG_OPTS}
 	${MAKE} -C ${KERNEL_SRC} oldconfig
+	# copy to allow building in parallel to kernel/module compilation without interference
+	rm -rf ${KERNEL_SRC_COPY}
+	cp -ar ${KERNEL_SRC} ${KERNEL_SRC_COPY}
 	touch $@
 
 .compile_mark: .config_mark
@@ -179,9 +182,6 @@ binary: install
 	rm -rf debian/${PVE_HEADER_PKG}
 	mkdir -p debian/${PVE_HEADER_PKG}/usr/src/linux-headers-${KVNAME}
 	install -m 0644 ${KERNEL_SRC}/.config debian/${PVE_HEADER_PKG}/usr/src/linux-headers-${KVNAME}
-	# copy to allow building in parallel to kernel/module compilation without interference
-	rm -rf ${KERNEL_SRC_COPY}
-	cp -ar ${KERNEL_SRC} ${KERNEL_SRC_COPY}
 	make -C ${KERNEL_SRC_COPY} mrproper
 	cd ${KERNEL_SRC_COPY}; find . -path './debian/*' -prune \
 	    -o -path './include/*' -prune \