Use correct executable name depending on OS #4269

[KnapSac]

• PR Description
  Use correct executable name depending on OS.

When unzipping the new version, we were always extracting lazygit. On Windows, the executable is named lazygit.exe. This PR fixes that issue.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

I'm not really sure how to test this (and as far as I can tell there are no integration tests for this feature yet). When attempting to use the feature from a version started using go run main.go it fails when trying to replace the current executable. I'm unsure whether this is due to something go run does, or whether that is Windows specific behavior. If I remember correctly, on Windows you can't replace the current executable, so perhaps that needs dedicated support on Windows.

Fixes #4269