False message on `igniter.install` task

**Describe the bug**
Choosing `n` on the `igniter.install` task returns "Successfully installed" even though the task was not performed (mix.exs didn't change as expected)

```
❯ mix igniter.install phoenix_test
compile ✔

Update: mix.exs

...|
33 33 | defp deps do
34 34 | [
35 + | {:phoenix_test, "~> 0.5"},
35 36 | {:igniter, "~> 0.5"},
36 37 | {:phoenix, "~> 1.7.18"},
...|

These dependencies should be installed before continuing. Modify mix.exs and install? [y/n] n

Successfully installed:

* phoenix_test
```

It's a minor issue but I can take a look on this one soon

Reproduce the issue with `mix igniter.install phoenix_test` and choose `n` at the prompt. Start by locating the `igniter.install` task and the code that prints the success message, then check how the declined `mix.exs` modification is reported. Done means declining installation no longer reports `phoenix_test` as successfully installed, with the existing task behavior covered by an appropriate test.

Written by the indexing model from the issue text.