fix(compute_tools): write_postgres_conf missing newline

[fffzlfk]

Problem

postgres_conf missing newline

Summary of changes

replace write! with writeln!

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [x] If it is a core feature, I have added thorough tests.
• [x] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• [x] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• [x] Do not forget to reformat commit message to not include the above checklist

[github-actions[bot]]

3006 tests run: 2891 passed, 0 failed, 115 skipped (full report)

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Flaky tests (1)

#### Postgres 15 - `test_pageserver_restarts_under_worload`: [release](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8248/9779773040/index.html#suites/542b1248464b42cc5a4560f408115965/a50e8a5eaa46712b/retries)

Code coverage* (full report)

• functions: 32.7% (6934 of 21213 functions)
• lines: 50.0% (54318 of 108580 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
12c2fc96834f59604b8ade5b9add28f1dce41ec6 at 2024-07-03T15:35:45.199Z :recycle:}