Ensure that clients can be configured through env vars only

```
(jumpstarter) printenv | grep JMP
JMP_NAME=pwallrab
JMP_ENDPOINT=<redacted>:443
JMP_NAMESPACE=jumpstarter-lab
JMP_TOKEN=<redacted>
(jumpstarter) jmp get exporters
Error: none of --client, --client-config is specified, and default config is not set
```

Also add a test for this specific functionality.