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	78.64M
workspace	122.03M
regression_4709	294.72M
ram_blowup_regression	2.44G
private-kernel-tail	208.79M
private-kernel-reset	862.88M
private-kernel-inner	307.69M
parity-root	174.43M

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.531s	-4%
regression_4709	0m1.568s	1%
ram_blowup_regression	0m17.165s	0%
private-kernel-tail	0m1.170s	-21%
private-kernel-reset	0m9.338s	1%
private-kernel-inner	0m2.370s	-1%
parity-root	0m0.899s	-23%
noir-contracts	2m48.732s	6%

[TomAFrench]

Blocked by noir-lang/noir_bigcurve#22