Fix soundness problem when matching `iso` variables #2861

Job	Run time
Lint bash, docker, markdown, and yaml	1m 41s
Validate GNU Docker image builds	43s
Validate GNU Docker release image builds	30s
Validate musl Docker image builds	20s
Validate musl Docker release image builds	22s
Validate Windows Docker release image builds	6m 36s
Validate Windows Docker image builds	13m 54s
Verify CHANGELOG is valid	5s
aarch64 Linux glibc	18m 8s
arm64 Apple Darwin	15m 2s
armhf Linux glibc	27m 58s
riscv64 Linux glibc	24m 41s
arm Linux glibc	28m 14s
x86-64 Linux glibc	20m 55s
x86-64 Linux musl	23m 27s
x86-64 Apple Darwin	22m 35s
x86-64 Windows MSVC	39m 15s
use dtrace	20m 55s
use pool_memalign	23m 47s
use runtimestats	22m 41s
	5h 11m 49s

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