litex_setup: Current --tag argument does not work

[nickyu42]

Related #903

The litex_setup.py setup script has the option to specify the a --tag argument to pin the version of LiteX, however this currently does not work as the https://github.com/litex-hub/litei2c repository does not have the requisite tags, so git rev-list -n 1 <tag> in git_checkout() fails.

I was trying to pin to 2023.12 using a previous version of litex_setup.py (without litei2c), but it auto updates itself to include the i2c core repo, so a workaround I am using right now is running the script in dev mode (passing --dev) to not auto update itself and setting GITHUB_ACTIONS=true so that init is not run in dev mode.