build_all: --ram mode (build scratch on tmpfs) for slow storage

Redirect the per-channel build scratch to /dev/shm and skip UPX, a big win when
building from a USB stick / SD card (I/O-bound on flash):
- TMPDIR + PIP_CACHE_DIR -> tmpfs
- PyInstaller: --workpath -> tmpfs (PYI_WORKPATH); UPX off via TESTIUM_NO_UPX
- Flatpak: build dir + ostree repo -> tmpfs (FLATPAK_BUILDDIR/REPODIR); the
  .flatpak-builder download cache stays on disk
- AppImage: bind-mount a tmpfs dir at the in-container AppDir path
  (APPIMAGE_APPDIR_TMPFS)
Scratch is freed on exit. Each build.sh honors the env vars with on-disk
defaults, so behavior is unchanged without --ram. With --ram, prefer --serial
on RAM-limited machines (flatpak+appimage are ~1 GB each).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package/pyinstaller/testium.spec

@@ -89,7 +89,9 @@ exe = EXE(
     debug=False,
     bootloader_ignore_signals=False,
     strip=False,
-    upx=True,
+    # UPX is CPU+IO heavy for a marginal size gain — build_all --ram sets
+    # TESTIUM_NO_UPX=1 to skip it (much faster on slow/flash storage).
+    upx=not os.environ.get("TESTIUM_NO_UPX"),
     upx_exclude=[],
     runtime_tmpdir=None,
     console=True,