server: use correct DB connection in create_work()

[davidpanderson]

A work generator can create and use its own DB connection.
Fix a bug in create_work() that incorrectly used the boinc_db connection
(which might not even be initialized).

Fixes #4032

[codecov]

Codecov Report

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

Project coverage is 10.53%. Comparing base (a1406d2) to head (e8a8a5f).
Report is 2 commits behind head on master.

Files	Patch %	Lines
tools/backend_lib.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5742   +/-   ##
=========================================
  Coverage     10.53%   10.53%           
  Complexity     1068     1068           
=========================================
  Files           279      279           
  Lines         35886    35886           
  Branches       8415     8415           
=========================================
  Hits           3780     3780           
  Misses        31712    31712           
  Partials        394      394           

Files	Coverage Δ
tools/backend_lib.cpp	0.00% <0.00%> (ø)