ci: Avoid subshell for executing HWCI_TEST_SCRIPT

Ensure that $HWCI_TEST_SCRIPT is an executable we can run ourselves, and
run that directly instead of invoking a subshell.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31110>

.gitlab-ci/run-shellcheck.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 
 CHECKPATH=".gitlab-ci"
+export SCRIPTS_DIR="${CHECKPATH}"
 
 is_bash() {
     [[ $1 == *.sh ]] && return 0