fix(agent): read /proc/uptime on linux instead of sysinfo(2) (#2180)

gopsutil's host.Uptime() calls the sysinfo(2) syscall. Inside an LXC
container lxcfs virtualizes /proc/uptime but cannot intercept a
syscall, so every container reported the host's uptime.

Reads /proc/uptime on linux and falls back to host.Uptime() if the file
is missing or unparseable, so other platforms are unchanged.

agent/uptime_stub.go

@@ -0,0 +1,10 @@
+//go:build !linux
+
+package agent
+
+import "github.com/shirou/gopsutil/v4/host"
+
+// getUptime returns the system uptime in seconds.
+func getUptime() (uint64, error) {
+	return host.Uptime()
+}