Do not let the shell wait for the command.

Otherwise when the command hangs, the shell would wait for it while keeping the stdout/err open.
This would freeze the fuzz test to wait for the pipe to close when using a separate Centipede process, even after Centipede ends.

PiperOrigin-RevId: 767726799

centipede/command.cc

diff --git a/centipede/command.cc b/centipede/command.cc
index f655217..d533bde 100644
--- a/centipede/command.cc
+++ b/centipede/command.cc
@@ -224,7 +224,7 @@
   {
     CENTIPEDE_FORK_SERVER_FIFO0="%s" \
     CENTIPEDE_FORK_SERVER_FIFO1="%s" \
-    %s
+    exec %s
   } &
   printf "%%s" $! > "%s"
 )sh";