Fix soundness problem when matching `iso` variables #2842

Job	Run time
Validate GNU Docker image builds	32s
Validate Windows Docker image builds	10m 43s
aarch64 Linux glibc	10m 41s
arm Linux glibc	10m 40s
armhf Linux glibc	10m 41s
riscv64 Linux glibc	10m 39s
Lint bash, docker, markdown, and yaml	1m 43s
Validate GNU Docker release image builds	31s
Validate musl Docker image builds	21s
Validate musl Docker release image builds	20s
Validate Windows Docker release image builds	7m 10s
Verify CHANGELOG is valid	6s
x86-64 Linux glibc	10m 39s
x86-64 Linux musl	10m 40s
arm64 Apple Darwin	10m 56s
x86-64 Apple Darwin	10m 49s
x86-64 Windows MSVC	10m 33s
use ${{ matrix.directives }}	1s
	1h 57m 45s

Provide feedback