Exclude darwin/386

`darwin/386` is dropped since go 1.15

```
--> darwin/386 error: exit status 2
Stderr: cmd/go: unsupported GOOS/GOARCH pair darwin/386
```

https://app.circleci.com/pipelines/github/sue445/gitpanda/1037/workflows/a60fc726-fbda-46e6-8bda-d3fdb3ff0b18/jobs/5190

So I dropped this.

mitchellh/gox#146 (comment)

Makefile

@@ -17,7 +17,7 @@ linux_amd64: $(SRCS)
 
 .PHONY: gox
 gox:
-	gox -ldflags=$(LDFLAGS) -output="bin/gitpanda_{{.OS}}_{{.Arch}}"
+	gox -ldflags=$(LDFLAGS) -osarch "!darwin/386" -output="bin/gitpanda_{{.OS}}_{{.Arch}}"
 
 .PHONY: zip
 zip: