docs(support): document nix as a tier 2 platform (#4130)

Clarify that Nix is a Tier 2 platform. We've gotten a few PRs for it,
so be explicit about its support status.

docs/support.md

diff --git a/docs/support.md b/docs/support.md
index 194cd35..fba88b8 100644
--- a/docs/support.md
+++ b/docs/support.md
@@ -69,8 +69,8 @@
   What is more, `windows_x86_64` is in this list, as we run tests in CI, but
   developing for Windows is more challenging, and features may come later to
   this platform.
-* Tier 2 - The rest of the platforms that may have a varying level of support, e.g.,
-  `linux_s390x`, `linux_ppc64le`, `windows_arm64`.
+* Tier 2 - Other platforms have varying levels of support, e.g.,
+  `linux_s390x`, `linux_ppc64le`, `windows_arm64`, `nix`.
 
 :::{note}
 Code to support Tier 2 platforms is allowed, but regressions will be fixed on a