wasm-builder: Use riscv32emac-unknown-none-polkavm.json target #13752

Job	Run time
check-dependency-rules	9s
check-fail-ci	13s
check-markdown	15s
check-workspace	20s
preflight / preflight	44s
preflight / ci-versions	1m 1s
check-rust-feature-propagation	2m 1s
fmt	1m 4s
check-toml-format	56s
check-umbrella	2m 20s
test-rust-features	2m 4s
All quick checks passed	1s
	11m 8s

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