scripts: name the mechanism in detect_supervisor's constraint 2, not a line number

Constraint 2 read "This script runs under `set -euo pipefail` (line 50), so an
unset variable is a loud failure." Two problems, both small and both the same
family as fleetd #494 — a comment that states the wrong reason.

The line number was stale: the `set` line is at 54, not 50. It was the only
line-number citation in the file, and a citation like that goes stale on the
next insert above it, silently, with nothing to catch it.

The mechanism was also misattributed. What makes an unset variable a loud
failure is `set -u`. Naming the whole `-euo pipefail` string invites the reader
to credit pipefail for it, which is the mistake fleet01 flagged on a different
cell this week: pipefail is insurance against a future pipeline stage, not what
catches the current shape.

Now names `set -u` and says where it is without a number, and records why the
number is gone so nobody adds one back.

Comment only. bash -n exit 0 under /bin/bash 3.2.57 and env bash 5.3.9;
scripts/test-redeploy-fleetd.sh exit 0 with 0 lines matching ^FAIL:.

scripts/redeploy-fleetd.sh

@@ -306,10 +306,12 @@ systemd_loaded() {
 #      never assigned to a global: a global set inside a `$( )` subshell dies with that subshell.
 #      This function packs BOTH values (kind and detail) onto that one stdout line, joined by
 #      $SUPERVISOR_DETAIL_SEP, and the caller unpacks them on its own side of the subshell boundary.
-#   2. This script runs under `set -euo pipefail` (line 50), so an unset variable is a loud
-#      failure. Do not add a `${VAR:-default}` anywhere downstream to paper over a value that
-#      should always be there — that hides a lost value instead of surfacing it (fleetd #497's
-#      defect class).
+#   2. This script runs under `set -u` (part of the `set -euo pipefail` at the top of the file), so
+#      an unset variable is a loud failure. Do not add a `${VAR:-default}` anywhere downstream to
+#      paper over a value that should always be there — that hides a lost value instead of
+#      surfacing it (fleetd #497's defect class). The mechanism is named rather than cited by line
+#      number on purpose: a line number in a comment goes stale on the next insert above it, and
+#      this one already had — it said line 50 while the `set` line was at 54.
 #   3. Every `case` on this function's return value needs an explicit final `*)` arm, chosen by
 #      whether that caller ACTS on the value (`die` — an unrecognised value must never be silently
 #      driven) or only DISPLAYS it (`echo`/`warn` and continue — a diagnostic must not go silent on