cmake: Fix unexpected token after &

In bash a '&&' cannot follow directly after a '&'. So a task that is
run in the background should be enclosed in (curly) braces.

This:
```
west build -b qemu_x86 -d build/server samples/net/sockets/echo_server\
 -- -DOVERLAY_CONFIG=overlay-qemu_802154.conf -DPCAP=capture.pcap
west build -t server -d build/server
```
works correctly after this fix.

Fixes #29793

Signed-off-by: Daniel Noom <ggatw@outlook.com>