Jenkinsfile: Fix UPX not available in PATH error.

Jenkinsfile

@@ -9,8 +9,7 @@ def binext(os) {
 
 def upx(file) {
   // Install upx
-  tool "UPX v3.91"
-
+  withEnv(["PATH+=${tool "UPX v3.91"}"]) {
     switch("${env.GOOS}.${env.GOARCH}") {
       case "linux.amd64":
       case "linux.386":
@@ -30,6 +29,7 @@ def upx(file) {
         echo "Skipping UPX compression as it is not supported for $goos/$goarch."
         break
     }
+  }
 }
 
 def withGoEnv(os, arch, f) {