Generate fish compatible docker-env hint

`fish`'s eval handling does not properly process multiple lines.

Instead the recommendation is to pipe the output to `source`.

This PR updates the usage hint of the `docker-env` command
when running on `fish`:

```
eval (minikube -p fish docker-env) => (minikube -p fish docker-env) | source
```

Ref: fish-shell/fish-shell#3993
Signed-off-by: Kevin Pullin <[email protected]>

Author: kppullin

cmd/minikube/cmd/docker-env_test.go

@@ -95,7 +95,7 @@ set -gx DOCKER_CERT_PATH "/certs"
 set -gx MINIKUBE_ACTIVE_DOCKERD "fish"
 
 # To point your shell to minikube's docker-daemon, run:
-# eval (minikube -p fish docker-env)
+# minikube -p fish docker-env | source
 `,
 			`set -e DOCKER_TLS_VERIFY
 set -e DOCKER_HOST

pkg/minikube/shell/shell.go

@@ -97,7 +97,7 @@ func generateUsageHint(sh, usgPlz, usgCmd string) string {
 `, usgPlz, usgCmd),
 		"fish": fmt.Sprintf(`
 # %s
-# eval (%s)
+# %s | source
 `, usgPlz, usgCmd),
 		"powershell": fmt.Sprintf(`# %s
 # & %s | Invoke-Expression