write manifest when writing run_results

[gshank]

resolves #2765

Description

Added a line to write the manifest at the same time that we write the run_results.json.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[gshank]

I didn't add a test because we already have a lot of tests for manifest.json and the only difference here is that it's written a bit later.

[jtcohen6]

Looks good to me! Nice straightforward addition.

From the issue:

TBD if this should be done a second time or if we should skip the current write that occurs after parsing finishes.

We can decide later if we want to remove the post-parsing manifest write, as part of our upcoming performance tuning.