refactor(ci): simplify sqlsmith workflow

[kwannoel]

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What's changed and what's your intention?

• Disable frontend tests. They fail after Unsupported function: array(int32, int32, int32, int32) -> list #12577.
• This will save cost, the frontend is still being fuzzed in main-cron, by the e2e fuzzing step.
• Re-enable sqlsmith tests for Pull Requests which contain changes to sqlsmith source files.
• Merge all sqlsmith workflows into main-cron.
  • Previously we separated it into sqlsmith-tests because sqlsmith is flaky sometimes, and we wanted to keep main-cron stable.
  • But I think we should merge it back to save costs (extra build step is required for sqlsmith tests).
  • Also easier to maintain just 2 instead of 3 workflows with sqlsmith.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[fuyufjh]

LGTM