Add lint to ensure substitutions use the full form

Check that any variable substitutions use the full ${VAR} form,
not just $VAR (but don't check for quoting yet).

etc/ci/lockfile_changed.sh

@@ -9,5 +9,5 @@ set -o nounset
 set -o pipefail
 
 diff="$(git diff -- */*/Cargo.lock)"
-echo "$diff"
-[[ ! $diff ]]
+echo "${diff}"
+[[ -z "${diff}" ]]

etc/ci/manifest_changed.sh

@@ -17,5 +17,5 @@ set -o pipefail
 ./mach test-wpt --manifest-update --binary= SKIP_TESTS > /dev/null
 
 diff="$(git diff -- tests/*/MANIFEST.json)"
-echo "$diff"
-[[ ! $diff ]]
+echo "${diff}"
+[[ -z "${diff}" ]]

etc/ci/upload_docs.sh

@@ -12,7 +12,7 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-cd "$(dirname $0)/../.."
+cd "$(dirname ${0})/../.."
 
 ./mach doc
 # etc/doc.servo.org/index.html overwrites $(mach rust-root)/doc/index.html

etc/ci/upload_nightly.sh

@@ -27,7 +27,7 @@ upload() {
 
 
 main() {
-    if [[ "$#" != 1 ]]; then
+    if [[ "${#}" != 1 ]]; then
         usage >&2
         return 1
     fi
@@ -58,4 +58,4 @@ main() {
     upload "${platform}" ${package} "${extension}"
 }
 
-main "$@"
+main "${@}"