ci: install WAMR for WASI standard-library jobs

[cpunion]

The standard-library workflow installs Wasmtime but never installs WAMR. Since LLGo #2697 made WAMR threads the default W32 backend, every W32-WASI package fails to launch with exec: "iwasm": executable file not found in $PATH. The same missing executable also fails the WASI GOROOT sentinels. The report in #28 contains 190 W32 package failures from this single setup omission.

Restore the checked-out LLGo revision's WAMR cache and run its pinned dev/build_iwasm.sh in both the W32-WASI acceptance jobs and the GOROOT sentinel job. The builder checks its source/patch build identity and exports the executable directory through GITHUB_PATH. Cache restoration is limited to WAMR; Wasmtime remains available for reference tooling.

Refs #28. The three independent J32-GoJS host-check failures also require the LLGo acceptance-child runner fix in xgo-dev/llgo#2718.

Validation:

• actionlint .github/workflows/std.yml and git diff --check.
• Ran the same dev/build_iwasm.sh installer with GITHUB_PATH set; it selected the matching cached binary and exported the expected directory.
• With that WAMR binary on PATH, the actual W32-WASI GOROOT runner passed bom.go and helloworld.go (the two previously failing sentinels).
• Ran the full W32-WASI audit driver for test (39 tests), test/go (242 tests), and test/llgoext (24 tests), selecting one whole package per shard (0, 9, and 17 of 201). All three packages and all 14 host checks passed, including builtin print, panic/repanic locations, invalid-finalizer diagnostics, and main-Goexit lifecycle. This was targeted validation of the affected execution paths; the complete 190-package W32 audit was not rerun locally.