Let agent pass filter expression string for builds

Drone agent accepts DRONE_FILTER env var

cmd/drone-agent/agent.go

@@ -32,6 +32,7 @@ func loop(c *cli.Context) error {
 		Labels: map[string]string{
 			"platform": c.String("platform"),
 		},
+		Expr: c.String("drone-filter"),
 	}
 
 	hostname := c.String("hostname")

cmd/drone-agent/main.go

@@ -48,6 +48,11 @@ func main() {
 			Name:   "platform",
 			Value:  "linux/amd64",
 		},
+		cli.StringFlag{
+			EnvVar: "DRONE_FILTER",
+			Name:   "drone-filter",
+			Usage:  "A filter expression used to restrict builds by label",
+		},
 		cli.IntFlag{
 			EnvVar: "DRONE_MAX_PROCS",
 			Name:   "max-procs",