Add flox install (#3656)

- **PR Description**

`lazygit` is available in Flox today, and seems to be working well.
Since Flox is related to Nix, I decided a good place would be right
below the Nix install section. 😉

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

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README.md

@@ -371,6 +371,16 @@ nix run nixpkgs#lazygit
 Or you can add lazygit to you configuration.nix in the environment.systemPackages section.
 More details can be found via NixOs search [page](https://search.nixos.org/).
 
+### Flox
+
+Lazygit can be installed into a Flox environment as follows.
+
+```sh
+flox install lazygit
+```
+
+More details about Flox can be found on [their website](https://flox.dev/).
+
 ### FreeBSD
 
 ```sh