ci/shellcheck: Don't overwrite SCRIPTS_DIR with relative path

shellcheck doesn't like it when we have relative paths and it's trying
to check if the files exist.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31602>

.gitlab-ci/run-shellcheck.sh

@@ -1,7 +1,6 @@
 #!/usr/bin/env bash
 
-CHECKPATH=".gitlab-ci"
-export SCRIPTS_DIR="${CHECKPATH}"
+SCRIPTS_DIR="$(realpath "$(dirname "$0")")"
 
 is_bash() {
     [[ $1 == *.sh ]] && return 0
@@ -21,4 +20,4 @@ while IFS= read -r -d $'' file; do
             exit 1
         fi
     fi
-done < <(find $CHECKPATH -type f \! -path "./.git/*" -print0)
+done < <(find "$SCRIPTS_DIR" -type f \! -path "./.git/*" -print0)