Add Cachix GH App to the org

[aduh95]

I've added in nodejs/node#60428 a binary cache to the test-shared workflow, and a secret to give GHA write access to that cache. @targos in nodejs/node#60315 (comment) was asking how to rotate the secret if need be, and indeed it's best I don't stay the only one with access to it.

Assuming no objections, the plan is:

• Grant Organizations and teams: Read-only access permission to Cachix GitHub App
• Grant @nodejs/tsc admin access to the cache.

[targos]

SGTM, thanks!

[aduh95]

I'm planing forward with the plan after the required 72 hours waiting time if I don't hear any objections:

admin/GITHUB_ORG_MANAGEMENT_POLICY.md

Lines 126 to 127 in e990d9c

	The request must be approved by at least two TSC voting members and
	be open for a minimum of 72 hours before landing.