ci: mipsel-softfloat — use mipsel-musl image + RUSTFLAGS=+soft-float

The image tag I assumed existed (`mipsel-musl-softfloat`) isn't
published by messense; docker-pull errored with "manifest unknown".
Available mipsel tags are just `mipsel-musl` (hardfloat) and the
regional mirrors of same.

Fix: use `messense/rust-musl-cross:mipsel-musl` (the standard
hardfloat image) and force soft-float code generation via
`RUSTFLAGS=-C target-feature=+soft-float` on top of the nightly
`-Z build-std=std,panic_abort` we were already using. build-std
recompiles libstd with the same RUSTFLAGS, so libstd itself comes
out soft-float even though the image's gcc/musl is hardfloat. We
don't link anything beyond libc for mhrv-rs (ring is pure-asm for
the crypto hot paths), so the fact that musl libm isn't soft-float
doesn't bite us.

Net result: the binary emits no hardware FP instructions, which is
what MT7621 actually needs.

.github/workflows/release.yml

@@ -150,16 +150,22 @@ jobs:
             cargo build --release --target aarch64-unknown-linux-musl --bin mhrv-rs
           sudo chown -R "$(id -u):$(id -g)" target
 
-      # OpenWRT MT7621 / mipsel-softfloat. The messense image tag
-      # `mipsel-musl-softfloat` ships a toolchain that emits soft-float
-      # insn exclusively — matches the MT7621's FPU-less reality.
-      # Requires Rust nightly + -Z build-std because mipsel is tier 3
-      # in the stable channel, which means no pre-built std.
+      # OpenWRT MT7621 / mipsel-softfloat. messense doesn't publish a
+      # `:mipsel-musl-softfloat` tag — the mipsel-musl image is
+      # hardfloat. We build soft-float anyway via
+      # `RUSTFLAGS=-C target-feature=+soft-float` + `-Z build-std` so
+      # libstd itself is recompiled to emit soft-float code. The
+      # gcc/musl shipping in the image is hardfloat but we never link
+      # anything more than libc (`ring` is pure asm for the crypto
+      # that matters), so musl's lack of softfloat libm doesn't bite.
+      # Requires nightly Rust since mipsel is Rust tier 3 in the
+      # stable channel — no prebuilt std.
       - name: Build CLI (mipsel-softfloat via docker)
         if: matrix.target == 'mipsel-unknown-linux-musl' && matrix.mipsel_softfloat == true
         run: |
           docker run --rm -v "$PWD":/src -w /src \
-            messense/rust-musl-cross:mipsel-musl-softfloat \
+            -e RUSTFLAGS='-C target-feature=+soft-float' \
+            messense/rust-musl-cross:mipsel-musl \
             sh -c "rustup toolchain install nightly --profile minimal --component rust-src && \
                    cargo +nightly build --release \
                      -Z build-std=std,panic_abort \