has_persistent_tasks: use add not dev

[asinghvi17]

In situations where the targeted package has a sources section this will try to dev the package. Pkg then tries to find the packages indicated in sources, which will all be local paths assuming a monorepo structure, except those don't exist - because this is the stripped tarball you get from a Pkg server and not the full repo structure.

Using add should "just fix" that.

[codecov]

Codecov Report

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

Project coverage is 79.48%. Comparing base (7bd9a70) to head (25225a3).

Files with missing lines	Patch %	Lines
src/persistent_tasks.jl	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7bd9a70) and HEAD (25225a3). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (7bd9a70)	HEAD (25225a3)
	16	6

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   86.91%   79.48%   -7.44%     
==========================================
  Files          11       11              
  Lines         512      502      -10     
==========================================
- Hits          445      399      -46     
- Misses         67      103      +36     

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[lgoettgens]

I am not quite sure if I understand the problem you are trying to fix. Could you create some minimal example with some detailed instructions on how to get to an error that is fixed by this PR? (I haven't yoused the sources feature myself yet, so I'll need a helping hand for setting up a minimal example)

As a side note: CI seems to be unhappy about this change. Julia version pre 1.10 just succeed because they skip the persistent task set in their testsuite