Closes FR-130

Builds compiled from source at a release tag report a non-dev release
channel, so they used to install the crash handler and write minidumps
into the shared logs directory. An official Zed launched later would
find and upload those dumps to Sentry, where they can never be
symbolicated (their debug UUID matches no uploaded debug files).

Gate the crash handler on having somewhere to send the dumps: either the
compile-time ZED_MINIDUMP_ENDPOINT baked into official builds, the
runtime ZED_MINIDUMP_ENDPOINT override, or an explicit
ZED_GENERATE_MINIDUMPS opt-in for debugging the crash pipeline locally.

Release Notes:

- N/A