Run 'go mod tidy' to ensure modules files are consistent with the req…

…uirements

This is a port of the xcaddy fix: caddyserver/xcaddy#47

Closes #9

builder.go

@@ -92,6 +92,12 @@ func (b Builder) Build(ctx context.Context, outputFile string) error {
 
 	log.Println("[INFO] Building k6")
 
+	// tidy the module to ensure go.mod and go.sum are consistent with the module prereq
+	tidyCmd := buildEnv.newCommand("go", "mod", "tidy")
+	if err := buildEnv.runCommand(ctx, tidyCmd, b.TimeoutGet); err != nil {
+		return err
+	}
+
 	// compile
 	cmd := buildEnv.newCommand("go", "build",
 		"-o", absOutputFile,