Use backticks inline instead of $(shell ...)
Use backticks inline instead of $(shell ...)

On the initial build run on a new system, the $(shell ...) variant fails
to provide any output for inline substitution and you end up with the
error:

```
gpg: `--verify-files' is not a valid long keyID
```

Using backticks will provide the desired output on the first run on a
system.

endocode/minikube,warmchang/minikube,kubernetes/minikube,dlorenc/minikube,dalehamel/minikube,dalehamel/minikube,kubernetes/minikube,kubernetes/minikube,r2d4/minikube,dongsupark/minikube,mikegrass/minikube,abbytiz/minikube,abbytiz/minikube,cu12/minikube,dims/minikube,cu12/minikube,dlorenc/minikube,aaron-prindle/minikube,endocode/minikube,dlorenc/minikube,dlorenc/minikube,warmchang/minikube,warmchang/minikube,dims/minikube,skipjaq/minikube,dlorenc/minikube,r2d4/minikube,abbytiz/minikube,dongsupark/minikube,mikegrass/minikube,skipjaq/minikube,warmchang/minikube,cu12/minikube,dongsupark/minikube,mikegrass/minikube,warmchang/minikube,aaron-prindle/minikube,kubernetes/minikube,dalehamel/minikube,warmchang/minikube,skipjaq/minikube,kubernetes/minikube,warmchang/minikube,aaron-prindle/minikube,endocode/minikube,kubernetes/minikube,kubernetes/minikube,r2d4/minikube,dlorenc/minikube,dims/minikube,dlorenc/minikube,dlorenc/minikube