build: fix check-xz for platforms defaulting to sh

5e80a9a1 introduced check-xz, using `[[ .. ]]` syntax, but this is a
bash builtin and some platforms default to `sh` when doing
`$(shell ...)` in Makefiles.

Fix is to make it sh friendly.

Ref: https://github.com/nodejs/node/pull/24551

PR-URL: https://github.com/nodejs/node/pull/24841
Refs: https://github.com/nodejs/node/pull/24551


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>