ensure correct exit when offline (#32)

without an internet connection, the script will:
- `exit` if sourced
- `return` if copied and pasted

Signed-off-by: Lucas Larson <LucasLarson@riseup.net>

update

@@ -1,5 +1,5 @@
 #!/usr/bin/env sh
-update='2023-07-16'
+update='2023-10-04'
 IFS="$(command printf -- ' \t\n|')" && IFS="${IFS%'|'}"
 command printf -- '\n\n                 .___       __\n __ ________   __\174 _\057____ _\057  \174_  ____\n\174  \174  \134____ \134 \057 __ \174\134__  \134\134   __\134\057 __ \134\n\174  \174  \057  \174_\076 \076 \057_\057 \174 \057 __ \134\174  \174 \134  ___\057\n\174____\057\174   __\057\134____ \174\050____  \057__\174  \134___  \076\n      \174__\174        \134\057     \134\057          \134\057\n a Lucas Larson production\n\n' >&2 && command sleep 1
 command printf -- '\360\237\223\241  verifying network connectivity' >&2
@@ -15,7 +15,7 @@ command printf -- '\n\n' >&2
 { command ping -c 1 -- one.one.one.one >/dev/null 2>&1 && command ping -c 1 -- 9.9.9.9 >/dev/null 2>&1; } || {
   update="${?-}"
   command printf -- 'No internet connection detected.\nAborting update.\n' >&2
-  return "${update:-1}"
+  case "${SHELL##*/}" in *"${0##*-}") return "${update:-1}";; *) exit "${update:-1}";; esac
 }
 set -- 'https://lucaslarson.net/update'
 command test "$({ command wget --output-document=- --quiet -- "${1-}" || command curl --location --silent -- "${1-}"; } 2>/dev/null | command sed -n -e '/update=/ {' -e 's/[^[:digit:]]//gp' -e 'q' -e '}')" -le "$(command printf -- '%s\n' "${update-}" | command sed -e 's/[^[:digit:]]//g')" || {