2.3.0+fork.113.occmdm

Fix CI crash on missing local.properties, rework nightly workflow

signingConfigs/buildTypes blocks evaluate unconditionally for every
task regardless of which variant is actually being built, so their
local.properties fallback (for CURRENT_DATE and the signing
passwords) was crashing any task - including assembleFdroidRelease,
which never even touches these values - on a checkout that doesn't
have that gitignored file, like F-Droid's buildserver or CI. Guards
the fallback behind a file-exists check instead of assuming it's there.

Also rewrites nightly-builds.yml:
- Fixes the push trigger branch (was "master", a leftover from before
  the fork - this repo's branch is "occm", so the workflow likely
  never actually ran on our own commits).
- Renames the moshidon-nightly.apk references to match this fork's
  actual output filename, occm-nightly.apk.
- Publishes to a rolling "nightly" pre-release instead of an ephemeral
  Actions artifact (which expires and needs a GitHub login to fetch),
  replacing the asset in place on every push, matching the style of
  fdroid-release.yml.

Verified assembleFdroidRelease/assembleNightly locally with
local.properties entirely absent, and confirmed the built APK's
filename matches what nightly-builds.yml now expects.