[WIP] [3006.x] Initial re-do of changes for _sync_grains only affecting masterless minions

[dmurphy18]

What does this PR do?

Ensures that grains are sync'd before loading grains and then compiling pillar, since there could be custom grains used in pillar files, which will throw rendering errors if not found, when in masterless mode on a salt-minion

Rework of accidentally closed PR 65792

What issues does this PR fix or reference?

Fixes: https://github.com/saltstack/salt/issues/65027

Previous Behavior

Would throw rendering errors if custom grains in salt://_grains were used in pillar files, salt-minion in masterless mode.

New Behavior

No rendering errors thrown if custom grains in salt://_grains were used in pillar files, and custom grains values available in pillar, and salt-minion in masterless mode.

Additional tests to be written ensuring use with a salt-master is unaffected.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [X] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [X] Tests written/updated

Commits signed with GPG?

Yes

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