Correctly find custom-built `llc` #2718

Job	Run time
Validate Windows Docker image builds	2m 7s
Validate GNU Docker image builds	31s
armhf Linux glibc	1h 20m 56s
riscv64 Linux glibc	1h 16m 4s
Lint bash, docker, markdown, and yaml	1m 41s
arm Linux glibc	1h 20m 44s
Validate GNU Docker release image builds	34s
Validate musl Docker image builds	28s
Validate musl Docker release image builds	25s
Validate Windows Docker release image builds	1m 5s
Verify CHANGELOG is valid	5s
x86-64 Linux glibc	20m 56s
aarch64 Linux glibc	22m 56s
x86-64 Linux musl	22m 7s
arm64 Apple Darwin	24m 26s
x86-64 Apple Darwin	31m 38s
x86-64 Windows MSVC	36m 41s
use dtrace	20m 36s
use pool_memalign	23m 48s
use runtimestats	21m 43s
	7h 49m 31s

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