tests/short-first-segment: fix uname(1) usage and skip message #504

Job	Run time
Build windows executable	49s
Build tarballs	33s
Build static musl binaries (amd64)	43s
Build static musl binaries (i386)	48s
Build static musl binaries (ppc64le)	3m 59s
Build static musl binaries (arm64v8)	5m 22s
Build static musl binaries (arm32v7)	3m 45s
Build static musl binaries (s390x)	4m 44s
Build static musl binaries (riscv64)	5m 8s
Test windows binaries	19s
Publish tarballs & binaries	0s
	26m 10s

Provide feedback