ci: correct OpenWrt MIPS-disabled comment to locate the actual blocker

Investigated the mipsel-unknown-linux-musl build of this branch on a
Linux/x86_64 host. ring 0.17, portable-atomic, and the fips codebase
itself all compile cleanly for that target — the AtomicU64 portability
work is already done. The actual blocker is in the nostr-relay-pool 0.44
transitive dep, which uses std::sync::atomic::AtomicU64 directly in
src/relay/{stats,ping,flags}.rs. Verified fixable with a 6-line
portable-atomic patch via a [patch.crates-io] shim during local testing.

Updating the comment so the next person looking at this matrix has the
right starting point.

No functional change.

.github/workflows/package-openwrt.yml

@@ -78,7 +78,9 @@ jobs:
             rust_target: aarch64-unknown-linux-musl
             rust_channel: stable
             # MT3000, MT6000, Flint 2, RPi 3/4/5
-          # MIPS disabled: 32-bit MIPS lacks AtomicU64; needs portable-atomic crate
+          # MIPS disabled: nostr-relay-pool 0.44 uses std::sync::atomic::AtomicU64
+          # directly (fips's own atomics already use portable_atomic). Re-enable
+          # once an upstream portable-atomic patch lands (or via [patch.crates-io]).
           # - build_arch: mipsel
           #   openwrt_arch: mipsel_24kc
           #   rust_target: mipsel-unknown-linux-musl