Fix soundness problem when matching `iso` variables #2843

Job	Run time
Lint bash, docker, markdown, and yaml	1m 41s
Validate musl Docker image builds	22s
arm64 Apple Darwin	13m 10s
aarch64 Linux glibc	21m 39s
armhf Linux glibc	1h 24m 20s
arm Linux glibc	1h 23m 17s
riscv64 Linux glibc	1h 18m 45s
Validate GNU Docker image builds	42s
Validate GNU Docker release image builds	41s
Validate musl Docker release image builds	20s
Validate Windows Docker image builds	4m 41s
Validate Windows Docker release image builds	4m 43s
Verify CHANGELOG is valid	9s
x86-64 Linux glibc	22m 42s
x86-64 Linux musl	25m 2s
x86-64 Apple Darwin	26m 3s
x86-64 Windows MSVC	36m 55s
use dtrace	20m 49s
use pool_memalign	23m 45s
use runtimestats	22m 11s
	7h 51m 57s

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