chore(ci): run noir-lang/big_curve tests on single thread

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR sets the RAYON_NUM_THREADS environment variable to 1 when running the bigcurve tests in run-external-checks. This should allow us to run all of the bigcurve tests in our CI to help avoid regressions such as those caused by #6685.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Peak Memory Sample

Program	Peak Memory
keccak256	78.15M
workspace	122.70M
regression_4709	423.26M
ram_blowup_regression	1.60G
private-kernel-tail	206.67M
private-kernel-reset	748.99M
private-kernel-inner	312.95M
parity-root	171.68M

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.370s	0%
regression_4709	0m0.846s	6%
ram_blowup_regression	0m15.028s	1%
rollup-base-public	3m24.426s	-15%
rollup-base-private	3m1.612s	-3%
private-kernel-tail	0m1.140s	-3%
private-kernel-reset	0m8.031s	-1%
private-kernel-inner	0m2.322s	-5%
parity-root	0m0.992s	2%
noir-contracts	2m53.055s	9%

[TomAFrench]

Blocked by noir-lang/noir_bigcurve#22

[TomAFrench]

Not necessary anymore as we can run tests on default settings.