fix(cron): align matrix home env var with /sethome conventio

[selika]

The cron delivery scheduler reads MATRIX_HOME_ROOM for the matrix home channel, but the /sethome command handler in gateway/run.py writes {platform.upper()}_HOME_CHANNEL — i.e. MATRIX_HOME_CHANNEL. All other 14 platforms in _HOME_TARGET_ENV_VARS use the unified _HOME_CHANNEL suffix; only Matrix is the outlier on _HOME_ROOM.

Effect: a Matrix cron job with deliver: "origin" and origin: null falls through to the home-channel fallback which checks MATRIX_HOME_ROOM — but that env var is never set by /sethome, so the fallback returns nothing and delivery is silently dropped.

This patch:

1. Renames the matrix entry in _HOME_TARGET_ENV_VARS to MATRIX_HOME_CHANNEL, matching what /sethome writes and aligning with the other platforms.
2. Adds MATRIX_HOME_CHANNEL → MATRIX_HOME_ROOM to _LEGACY_HOME_TARGET_ENV_VARS so any existing deployments that manually set MATRIX_HOME_ROOM continue to work via the legacy fallback path.

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[teknium1]

Thanks for the diagnosis @selika — your bug analysis was correct (Matrix /sethome was writing to an env var nothing read on startup), but the fix was at the wrong side. Aligning the cron reader to match the broken /sethome write would have broken the downstream contract: gateway/config.py:1177 reads MATRIX_HOME_ROOM, the setup wizard writes MATRIX_HOME_ROOM, tests/gateway/test_matrix.py fixtures use MATRIX_HOME_ROOM, and the env-vars documentation lists MATRIX_HOME_ROOM. The correct fix was to make /sethome honor those conventions.

Fixed at the write site via PR #18272 (salvaged from @mikeyobrien's earlier #12698), which also covers email's EMAIL_HOME_ADDRESS — same bug class on the other outlier platform. Credit to you for catching the user-visible symptom and to @mikeyobrien for the write-side fix.

#18272