Fix timezone step in mt-record-screenshots.yml for bike transit agencies

[Copilot]

The timezone step only read from gtfs_rts_values_gen.xml and hard-failed (exit 1) when the file or field was absent — breaking bike agencies that don't have this file.

Changes

• Moved xmllint install check before any XML lookups (deduplicated)
• Restructured as if/elif/else to clearly handle all three cases:
  • if gtfs_rts_values_gen.xml exists → read gtfs_rts_timezone (must be present, or exit 1)
  • elif bike_station_values.xml exists → read bike_station_timezone (optional — no exit 1 if absent, falls back to UTC via || 'UTC' in the emulator step)
  • else → neither file found → exit 1

# Resolution order:
# 1. gtfs_rts_values_gen.xml → gtfs_rts_timezone        (required if file present)
# 2. bike_station_values.xml → bike_station_timezone     (optional if file present)
# 3. UTC fallback (handled by TZ: ${{ steps.timezone.outputs.TIMEZONE || 'UTC' }})
# 4. All other cases → exit 1 (error)

Original prompt

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This section details on the original issue you should resolve

<issue_title>mt-record-screenshots.yml > compat w/ bike transit agency</issue_title>
<issue_description>shared-overwrite/.github/workflows/mt-record-screenshots.yml not compatible with bike transit agencies (timezone step)

https://github.com/mtransitapps/ca-quebec-a-velo-bike-android/actions/runs/23488844032/job/68351336280

Try reading timezone from app-android/src/main/res/values/bike_station_values.xml string of bike_station_timezone but it should be optional in this case since this field is not mandatory for bike agencies. Fallback to UTC timezone.

xmllint --xpath "string(//resources/string[@name='bike_station_timezone']/text() app-android/src/main/res/values/bike_station_values.xml
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes mt-record-screenshots.yml > compat w/ bike transit agency #625

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.