fix(build): Windows builds has no .exe extension (#21)

Windows builds are failing with error:
'Ignoring plugin match D:\a\packer-plugin-upcloud\packer-plugin-upcloud\builder\upcloud\packer-plugin-upcloud, no exe extension'

This was caused by latest change in make build

Author: peknur

Makefile

@@ -25,7 +25,7 @@ lint:
 	golint .
 
 build:
-	go build -v -o $(BINARY)
+	go build -v
 
 install: build
 	@mkdir -p ~/.packer.d/plugins