Let a host that needs no launcher run its own smoke test

Every macOS leg of the executor has been dying since the Rosetta gate
landed, after a complete and valid build: the SDK is finalized, the
static validation passes, the toolchain contract passes, and then
build-host.sh exits with `run[@]: unbound variable` and stages nothing.
The upload finds an empty out/ and the Intel host, which needs the arm64
SDK to cross from, fails behind it. arm64-apple-darwin is a required
host, so no snapshot could publish at all.

The launcher prefix is an array that is empty for every host that runs
its own binaries -- as the comment above it says -- and macOS's
/bin/bash is 3.2, where expanding an empty array under `set -u` is an
error rather than nothing:

    $ /bin/bash -c 'set -u; run=(); "${run[@]}" echo hola'
    /bin/bash: run[@]: unbound variable
    $ /bin/bash -c 'set -u; run=(); ${run[@]+"${run[@]}"} echo hola'
    hola

build_and_stage() already carries the tolerant form, and the reason for
it, three lines below.

Author: frangarcj

scripts/ci/build-host.sh

@@ -157,12 +157,15 @@ smoke_test_bootstrap() {
 	local -a run=(${2:-})
 	digest=$(awk '{print $1}' "$bootstrap_archive.sha256")
 	install_root="$PWD/bootstrap-installed"
+	# ${run[@]+...}: macOS's /bin/bash is 3.2, where expanding an empty array
+	# under set -u is an unbound-variable error -- and empty is the normal
+	# case here, for every host that runs its own binaries.
 	VITASDK_BOOTSTRAP_ARCHIVE="$bootstrap_archive" VITASDK_BOOTSTRAP_SHA256="$digest" \
-		"${run[@]}" build/vitasdk/share/vdpm/bootstrap-vitasdk.sh --install-dir "$install_root"
-	VITASDK="$install_root" "${run[@]}" "$install_root/bin/vdpm" --help >/dev/null
+		${run[@]+"${run[@]}"} build/vitasdk/share/vdpm/bootstrap-vitasdk.sh --install-dir "$install_root"
+	VITASDK="$install_root" ${run[@]+"${run[@]}"} "$install_root/bin/vdpm" --help >/dev/null
 	# vdpm ships pacman under libexec/vdpm, not bin/.
-	"${run[@]}" "$install_root/libexec/vdpm/pacman" --version >/dev/null
-	"${run[@]}" "$install_root/bin/arm-vita-eabi-gcc" --version
+	${run[@]+"${run[@]}"} "$install_root/libexec/vdpm/pacman" --version >/dev/null
+	${run[@]+"${run[@]}"} "$install_root/bin/arm-vita-eabi-gcc" --version
 }
 
 # Builds against $stage1_dir if set, then stages outputs plus provenance.