Fix installer to gracefully handle systemd not running in containers (#46068)

## What does this PR do?

Fixes the datadog-installer to gracefully handle environments where systemd is not running (e.g., containers, non-systemd init systems).

## Motivation

The installer was failing when attempting to execute `systemctl` commands on systems where systemd is installed but not running, which is common in containers.

## Changes

Added `IsRunning()` checks to systemd operations:
- `StartUnit()` - skips start if systemd not running
- `RestartUnit()` - skips restart if systemd not running  
- `EnableUnit()` - skips enable if systemd not running
- `Reload()` - skips daemon-reload if systemd not running

These functions now log at INFO level and return success (nil) when systemd is not running, allowing installation to continue normally.

## Testing

- Compilation verified
- Linters passed (0 issues)

Co-authored-by: baptiste.foy <baptiste.foy@datadoghq.com>

Author: BaptisteFoy

pkg/fleet/installer/packages/service/systemd/systemd.go

@@ -70,20 +70,44 @@ func StopUnit(ctx context.Context, unit string, args ...string) error {
 
 // StartUnit starts a systemd unit
 func StartUnit(ctx context.Context, unit string, args ...string) error {
+	running, err := IsRunning()
+	if err != nil {
+		return err
+	}
+	if !running {
+		log.Infof("Installer: systemd not running, skipping start of %s", unit)
+		return nil
+	}
 	args = append([]string{"start", unit}, args...)
-	err := telemetry.CommandContext(ctx, "systemctl", args...).Run()
+	err = telemetry.CommandContext(ctx, "systemctl", args...).Run()
 	return handleSystemdSelfStops(err)
 }
 
 // RestartUnit restarts a systemd unit
 func RestartUnit(ctx context.Context, unit string, args ...string) error {
+	running, err := IsRunning()
+	if err != nil {
+		return err
+	}
+	if !running {
+		log.Infof("Installer: systemd not running, skipping restart of %s", unit)
+		return nil
+	}
 	args = append([]string{"restart", unit}, args...)
-	err := telemetry.CommandContext(ctx, "systemctl", args...).Run()
+	err = telemetry.CommandContext(ctx, "systemctl", args...).Run()
 	return handleSystemdSelfStops(err)
 }
 
 // EnableUnit enables a systemd unit
 func EnableUnit(ctx context.Context, unit string) error {
+	running, err := IsRunning()
+	if err != nil {
+		return err
+	}
+	if !running {
+		log.Infof("Installer: systemd not running, skipping enable of %s", unit)
+		return nil
+	}
 	return telemetry.CommandContext(ctx, "systemctl", "enable", unit).Run()
 }
 
@@ -133,6 +157,14 @@ func WriteUnitOverride(ctx context.Context, unit string, name string, content st
 
 // Reload reloads the systemd daemon
 func Reload(ctx context.Context) (err error) {
+	running, runningErr := IsRunning()
+	if runningErr != nil {
+		return runningErr
+	}
+	if !running {
+		log.Infof("Installer: systemd not running, skipping daemon-reload")
+		return nil
+	}
 	return telemetry.CommandContext(ctx, "systemctl", "daemon-reload").Run()
 }