This fixes #14818.

The change in #14055 broke the tasks in `zsh` (and I suspect in `bash`,
`sh` too), because what was executed was NOT

    $ go test . -run '^TestThis$'

but instead this:

    $ go test . -run \'^TestThis$\'

And in `zsh` this means that `'` is part of the argument passed to `go`,
which means the targeting string is wrong.

Since the problem in `fish` doesn't seem to be the `^` but the `$`, we
can only escape that, which makes the escaped string work in `zsh` and
`fish` and `bash` (in which I've tested this change here)

Release Notes:

- go: Fix running single tests by changing the quoted expression in the
`go test` command to work again in `bash`, `zsh`, etc.
([#14818](https://github.com/zed-industries/zed/issues/14818))