Stop using a parallel sort in disjoint utils (backport #15410)

[mergify]

Summary

This commit switches the use of par_sort_by_unstable() to the serial sort_by_unstable() implementation in the disjoint layout module. While the parallel sort is faster and has some heuristics to limit the multithreaded usage when the overhead isn't worth the tradeoff, the use of multithreading from Python is potentially fraught when mixing with multiprocessing in Python. For this reason all of our multithreaded calls are supposed to guard against running in parallel based on env vars that indicate the rust code is being executed in a multiprocessing context. However the parallel sorts were missing this check and in this case the overhead of checking we can run in multiple threads will likely outweigh any speedup from a parallel sort. So this commit just opts to drop the parallel sort and avoid the potential complications.

Details and comments

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This is an automatic backport of pull request #15410 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of cae0817 has failed:

On branch mergify/bp/stable/2.2/pr-15410
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Changes to be committed:
	new file:   releasenotes/notes/fix_multiprocessing_with_multithreading_bug-948c7ade10f66c60.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   crates/transpiler/src/passes/disjoint_layout.rs

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[coveralls]

Pull Request Test Coverage Report for Build 19878305745

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.02%) to 88.441%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	93.63%
crates/circuit/src/parameter/symbol_expr.rs	2	73.19%
crates/qasm2/src/lex.rs	4	92.27%
crates/qasm2/src/parse.rs	6	97.56%

Totals
Change from base Build 19577979235:	0.02%
Covered Lines:	92414
Relevant Lines:	104492

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💛 - Coveralls