ci: install release Rust targets explicitly

[mulhamna]

Summary

• install each release matrix Rust target with rustup target add before building
• keep the existing musl C toolchain install step for Linux static binaries

Why

Release v0.18.0 failed during the musl build with:

error[E0463]: can't find crate for `core`
= note: the `x86_64-unknown-linux-musl` target may not be installed
= help: consider downloading the target with `rustup target add x86_64-unknown-linux-musl`

Installing the target in an explicit step makes the release workflow fail-fast at target setup and avoids depending on implicit action behavior.

Test plan

• go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/release.yml
• python3 structural check for the edited workflow block
• rustup target add x86_64-unknown-linux-musl
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• attempted cargo build --locked --release --target x86_64-unknown-linux-musl; local blocker is missing x86_64-linux-musl-gcc, which the workflow installs via musl-tools before build