Problem: The libjs-test262 job was failing — often — with “cargo is not
installed for the toolchain 1.96.0”, while building the Rust crates.
Cause: The job ran in an older CI image lacking 1.96.0 — making runtime
rustup try to auto-install 1.96.0 the first time a proxied cargo ran.
And because the build compiles multiple Rust crates in parallel, several
cargo invocations hit the missing toolchain at once — and each started a
rustup install of the same toolchain. And rustup apparently isn’t safe
against concurrent installs: Its in-place component swap shuffles files
out to a temp directory, and a crate that reads the toolchain mid-swap
sees rustc — but not cargo. Whether it corrupts depends on timing — so
the failure is intermittent, and a re-run usually “fixes” it.
Fix: Update the job to the current ladybird-ci:2026.06.02 image — which
already has 1.96.0 (with cargo) baked in — so no runtime install happens
and there’s nothing to race. As an additional guard, also install the
pinned toolchain *explicitly*, before the parallel build: so any install
happens exactly once — and serially — rather than being raced.
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