Installer keeps the user's TMPDIR #313

Part of #89.

Part of #311 (Group B, installer and update). Owner: not assigned. Task base and PR target: develop. Start after #312 merges (same file).

Intended behavior

The installer uses its own scratch directory, and every program it starts sees the user's original TMPDIR.

Current behavior

• scripts/install.sh:1093 sets TMPDIR=$(mktemp -d). The traps at lines 1094 and 1205 remove that directory when the installer exits.
• macOS exports TMPDIR by default, so the changed value reaches every child. These include elastos setup and the Home launch in finish_install (line 891).
• The update controller captures TMPDIR in the launch environment it keeps for later Runtime starts (elastos/crates/elastos-server/src/update_controller.rs:60, 116).
• The first Home session can therefore hold a temporary-directory path that the installer deletes when it exits.

Implementation notes

Rename the scratch variable, for example INSTALL_SCRATCH, everywhere in install.sh, and keep TMPDIR unchanged. publish-release.sh has its own TMPDIR handling and stays as it is.

Done means

• install.sh assigns no value to TMPDIR. Test: a policy check (grep -n '^TMPDIR=' scripts/install.sh is empty) in install-bootstrap-test.py. Proof: https://github.com/Elacity/elastos-runtime/blob/aa3d61e4478a1a1a63dbef0dae97c23a0f9f4851/scripts/install-bootstrap-test.py#L1579
• Children see the caller's TMPDIR. Test: run the installer fixture with TMPDIR=/custom/path, and assert that the stub elastos setup receives /custom/path. Proof: https://github.com/Elacity/elastos-runtime/blob/aa3d61e4478a1a1a63dbef0dae97c23a0f9f4851/scripts/install-bootstrap-test.py#L897-L898
• The macOS update hop in CI passes. Proof: https://github.com/Elacity/elastos-runtime/actions/runs/38241637563/job/114787675815