exec: don't panic on rootless raw_exec tasks (#26401)

the executor dies, leaving an orphaned process still running.

the panic fix:
 * don't `panic()`
 * and return an empty, but non-nil, func on cgroup error

feature fix:
 * allow non-root agent to proceed with exec when cgroups are off

drivers/shared/executor/executor_linux.go

@@ -7,7 +7,6 @@ package executor
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -766,7 +765,7 @@ func (l *LibcontainerExecutor) configureCgroups(cfg *runc.Config, command *ExecC
 
 	cg := command.StatsCgroup()
 	if cg == "" {
-		return errors.New("cgroup must be set")
+		return fmt.Errorf("configureCgroups: %w", ErrCgroupMustBeSet)
 	}
 
 	// // set the libcontainer hook for writing the PID to cgroup.procs file