Fix occasional `alloc` not found error in `format_runtime_string!` #7873

Job	Run time
check-dependency-rules	11s
set-image	10s
check-fail-ci	13s
check-workspace	18s
check-markdown	15s
check-rust-feature-propagation	1m 52s
fmt	1m 3s
check-umbrella	2m 4s
check-toml-format	55s
test-rust-features	1m 55s
	8m 56s

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