Fix "unconfigured TLS" panic in db::oneoff_async_connection() fn

[Turbo87]

The enqueue-job admin tool has been silently failing on our staging environment since #9632 was merged. This PR fixes it by reusing our establish_async_connection() fn, which sets up the TLS configuration correctly.

It is a bit unforunate that this wasn't caught earlier though. We could potentially look into using TLS for the database connection on CI, but we'd also need to write tests for all of the admin tools (which we should do anyway...).

It is also a bit surprising that the Heroku Scheduler, which regularly uses this admin tool to enqueue regular tasks, didn't report any warnings about the tool crashing all the time, but I guess there is not much we can do on that front :-/

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.71%. Comparing base (5673bdd) to head (cc93677).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/db.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9674   +/-   ##
=======================================
  Coverage   88.71%   88.71%           
=======================================
  Files         287      287           
  Lines       29339    29339           
=======================================
  Hits        26028    26028           
  Misses       3311     3311           

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