Fix examples/gateway/aws/setup.sh aborting on stock macOS bash 3.2

[Yyunozor]

setup.sh line 66 uses ${DIST_SHA256,,}, a bash 4 case-modification
expansion. macOS ships bash 3.2 as /bin/bash, so unless a newer bash is
earlier on PATH, #!/usr/bin/env bash resolves to 3.2 and the script aborts
there — before reaching its own argument checks. The line is unconditional, so
it fires whether or not DIST_URL/DIST_SHA256 are set. bash -n passes; the
failure is runtime-only.

From the repo root on stock macOS:

$ /bin/bash examples/gateway/aws/setup.sh
examples/gateway/aws/setup.sh: line 66: ${DIST_SHA256,,}: bad substitution

The replacement lowercases the same digests with tr, pinned to LC_ALL=C so
BSD tr cannot fail the script on a stray non-UTF-8 byte under set -e. With
it, the same bash 3.2 run reaches the normal path and prints
ERROR: AWS_REGION is not set. The comment moves above the line to keep the
block's comment column.

This is the only bash 4 construct I found in the file; the GCP example next
door has none. Portability is already an explicit concern here — see the
sha_of() comment on line 127.