`locale-gen <name>` is an Ubuntu extension; plain Debian's locale-gen
takes no arguments and only reads /etc/locale.gen, so the previous fix
generated nothing and makensis kept aborting with exit 84 / "FATAL: main
argv conversion failed!" under the runner's C/POSIX locale.
Write the entry into /etc/locale.gen, run locale-gen with no arguments,
and assert `locale -a` lists it — a missing locale now fails the install
step with a clear message instead of dying four minutes later inside
electron-builder.
build-release.ps1 sets LC_ALL/LANG itself before packaging a Windows
target on a Linux host, picking a real UTF-8 locale out of `locale -a`.
The electron-builder child then inherits it however the script was
started, including a runner that drops step env.
Also adds a non-fatal probe step that runs the cached makensis with
-VERSION under en_US.UTF-8 and under C, so a failure that is *not* the
locale is distinguishable from one that is without another blind fix.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>