Add flox install

[bryanhonof]

• 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)
• [x] Tests have been added/updated (see here for the integration test guide)
• [x] Text is internationalised (see here)
• [x] Docs have been updated if necessary
• [x] You've read through your own file changes for silly mistakes etc