ci: mipsel-softfloat — fix rustup upgrade-in-place error

The messense/rust-musl-cross:mipsel-musl image ships a pre-installed
nightly Rust in a state rustup can't cleanly upgrade over — the
in-place upgrade errors with

    error: failure removing component 'clippy-preview-x86_64-unknown-linux-gnu',
    directory does not exist: 'share/doc/clippy/README.md'

because the prior install is missing files rustup expects to delete.
Workaround: uninstall nightly first, then install fresh with the
minimal profile, then add rust-src as a separate step.

continue-on-error keeps this experimental target from blocking the
release — landing the fix on main so the NEXT tag attempt gets a
working mipsel artifact, without spinning up yet another retag dance
for v1.1.0.

.github/workflows/release.yml

@@ -166,7 +166,16 @@ jobs:
           docker run --rm -v "$PWD":/src -w /src \
             -e RUSTFLAGS='-C target-feature=+soft-float' \
             messense/rust-musl-cross:mipsel-musl \
-            sh -c "rustup toolchain install nightly --profile minimal --component rust-src && \
+            sh -c "set -eux; \
+                   # The image ships with a pre-installed nightly that rustup \
+                   # can't cleanly upgrade — the expected \`clippy-preview/share/doc/clippy/README.md\` \
+                   # is missing, which fails the in-place upgrade \
+                   # (error: failure removing component 'clippy-preview...'). \
+                   # Nuke it first, then install fresh with only the profile \
+                   # bits we actually use. \
+                   rustup toolchain uninstall nightly 2>/dev/null || true; \
+                   rustup toolchain install nightly --profile minimal; \
+                   rustup component add rust-src --toolchain nightly; \
                    cargo +nightly build --release \
                      -Z build-std=std,panic_abort \
                      --target mipsel-unknown-linux-musl \