Making run_tests.sh user friendly with -j and better OMP_NUM_THREADS support

[Renaud-K]

My motivation is to make run_tests.sh usable on local machines by developers. I ran in a couple issues trying to do that.

1. I typically already have OMP_NUM_THREADS set in my environment so that lit -j can be called directly. In this scenario nproc returns OMP_NUM_THREADS which defeats our purpose.
2. As a user I may want some control of how many cores are used, so I have also added -j support to overwrite nproc.

[github-actions]

CI Summary (push) — ✅ passed

Run #29608527002 · ✅ 6 · ⏩ 7 · ❌ 0 · ⛔ 0

Top-level jobs (13)

Job	Result
binaries	⏩ skipped
build_and_test	✅ success
config_devdeps	✅ success
config_source_build	⏩ skipped
config_wheeldeps	✅ success
devdeps	✅ success
docker_image	⏩ skipped
gen_code_coverage	⏩ skipped
metadata	✅ success
python_metapackages	⏩ skipped
python_wheels	⏩ skipped
source_build	⏩ skipped
wheeldeps	✅ success

⏩ Skipped jobs (7) — intentionally skipped on PR builds; run on merge_group / workflow_dispatch

Job
binaries
config_source_build
docker_image
gen_code_coverage
python_metapackages
python_wheels
source_build

All sub-jobs (42) — every matrix leg, with links

Job	Status	Link
Build and test (amd64, gcc12, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Python)	✅ success	view
Build and test (amd64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
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CI Summary	❔ in_progress	view
Configure build (devdeps)	✅ success	view
Configure build (source_build)	⏩ skipped	view
Configure build (wheeldeps)	✅ success	view
Create CUDA Quantum installer	⏩ skipped	view
Create Docker images	⏩ skipped	view
Create Python metapackages	⏩ skipped	view
Create Python wheels	⏩ skipped	view
Gen code coverage	⏩ skipped	view
Load dependencies (amd64, gcc12) / Caching	✅ success	view
Load dependencies (amd64, gcc12) / Finalize	✅ success	view
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Load dependencies (amd64, llvm) / Caching	✅ success	view
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Load dependencies (arm64, llvm) / Caching	✅ success	view
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Load wheel dependencies (arm64, 12.6) / Finalize	✅ success	view
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Load wheel dependencies (arm64, 13.0) / Caching	✅ success	view
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Required check	Status	Link
Build and test (amd64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, llvm, openmpi) / Dev environment (Python)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (arm64, llvm, openmpi) / Dev environment (Python)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Debug)	✅ success	view
Build and test (amd64, gcc12, openmpi) / Dev environment (Python)	✅ success	view