clean_checkout input to handle cleaning after checkout

[aliencaocao]

clean_checkout input

• default: true
• If true, perform a clean checkout; if false, skip cleaning. Cleaning will remove all existing local files not in the repository during checkout. If you use utilities like pyinstaller-versionfile, set this to false.

This fixes issue of user wanting to use their own version text file but it is being cleaned by the checkout and very hard to debug.
See aliencaocao/netease_cloudmusic_discord_rpc#4

[sayyid5416]

@aliencaocao Add this feature as optional. Add an input which controls if clean will be True or False

[aliencaocao]

@sayyid5416 is this ok?

[sayyid5416]

add in readme and other places as well, wherever necessary. I'll check later. Thanks for contribution btw.

[aliencaocao]

im not sure where else is needed except for readme. just updated it

[sayyid5416]

By default, clean should be true as it's true by default in actions/checkout

[aliencaocao]

done

[sayyid5416]

Do you need to add anything else to this PR?

[aliencaocao]

Nope