Include upgrade instructions in `run.linkerd.io/public/install`

Nobody has claimed this yet.

This is a feature request that came from discussion in [#3018 in the Linkerd2 repo](https://github.com/linkerd/linkerd2/issues/3018).

Currently, when a user runs `curl https://run.linkerd.io/install-edge | sh`, the confirmation text does not mention `linkerd upgrade`. This can be confusing as the "Now run:" spelled out in the confirmation text will not work for users if they already have Linkerd installed.

`run.linkerd.io/public/install` should have these changes:

When a user runs `curl https://run.linkerd.io/install-edge | sh`

* Check for linkerd in the user's path when they download Linkerd
* If linkerd is NOT in the user's path, add link to upgrade docs at the end of output
* If linkerd IS in the user's path, add link to upgrade docs before the "Now run:" section.

```bash
Linkerd was successfully installed 🎉

Upgrading? https://linkerd.io/2/tasks/upgrade/

Installing for the first time? Add the linkerd CLI to your path with:

export PATH=$PATH:$HOME/.linkerd2/bin

[rest of output stays the same]
```

Start at run.linkerd.io/public/install and inspect the output produced by `curl https://run.linkerd.io/install-edge | sh`. Verify the first-install and existing-CLI cases by checking whether `linkerd` is on PATH; done means the upgrade URL appears in the requested position while the remaining output stays unchanged.

Written by the indexing model from the issue text.