experiments: `core.experiments` config setting overridden in certain situations

[pmrowla]

If core.experiments is set in .dvc/config and not committed, experiments support will end up being disabled when we do reset --hard during experiments checkout. This also affects .dvc/experiments being added to .gitignore.

Temporary workarounds:

• Use dvc config --local when setting core.experiments true
• git commit changes to .dvc/config after doing dvc config core.experiments true, and before running any other experiments related commands

[pmrowla]

In the long run, experiments checkout behavior will need to be smarter than the POC git reset --hard implementation, and this issue should be resolved when that happens.