Improve voltage warning lines in TinyPilot logs (#1544)

Resolves #1495

Parses the output of `journalctl` into a simple “yes” or “no” result. If
the result is “yes”, the latest voltage warning is also included in the
output.

## Parsing the output of `journalctl`

I initially considered piping the output of `journalctl` into `grep` for
readability, but getting the correct exit code as well as just the
latest voltage warning was quite awkward with this approach. I therefore
settled on using just the filtering and output options that are built
into `journalctl`:

- The `-q` argument is used to suppress the unwanted `journalctl` header
text.
- The `-r` argument causes `journalctl` to output from newest to oldest.
- The `-n 1` argument causes `journalctl` to stop after outputting a
single line.
- The `-g "voltage"` argument limits output to only lines that contain
the word “voltage”.

The end result of this combination of arguments is that `journalctl`
will efficiently output only the most recent voltage warning. In
addition, the exit status of the command reflects whether or not any
matching lines were found, which allows for a simple `if <command>` test
to be used.

Short arguments were used for consistency with the majority of the rest
of the `collect-debug-logs` script.
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debian-pkg/opt/tinypilot-privileged/scripts/collect-debug-logs

@@ -148,9 +148,13 @@ print_info "Checking throttled state..."
 
 print_info "Checking for voltage issues..."
 {
-  echo "voltage logs"
-  journalctl --catalog --pager-end | grep --ignore-case "voltage"
-  printf "\n"
+  printf "Voltage issues: "
+  if LAST_VOLTAGE_LINE="$(journalctl --quiet --reverse --lines=1 --grep='voltage')" ; then
+    printf "yes - %s" "${LAST_VOLTAGE_LINE}"
+  else
+    printf "no"
+  fi
+  printf "\n\n"
 } >> "${LOG_FILE}"
 
 print_info "Checking TinyPilot settings..."