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/build.sh

@@ -2,11 +2,15 @@
 
 SCRIPT_DIR=$(realpath $( dirname "$0"))
 
-rm -r "${SCRIPT_DIR}/build" "${SCRIPT_DIR}/dist"
+rm -rf "${SCRIPT_DIR}/build" "${SCRIPT_DIR}/dist"
 
 pwd=$(pwd)
 cd ${SCRIPT_DIR}
-pyinstaller testium.spec
+# PYI_WORKPATH (set by build_all --ram) puts the big intermediate build tree on
+# tmpfs; dist/ stays local so build_all can collect the binary.
+WORKARG=""
+[ -n "$PYI_WORKPATH" ] && WORKARG="--workpath $PYI_WORKPATH"
+pyinstaller $WORKARG testium.spec
 RESULT=$?
 if [ -n "$1" ] && [ "$1" = "install" ]; then
     if [ $RESULT -eq 0 ]; then