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/flatpak/build.sh

@@ -7,11 +7,15 @@
 
 set -e
 
-# Build + install local
-flatpak-builder --user --verbose --force-clean --install --repo=repo build org.testium.Testium.yaml
+# Build + install local. FLATPAK_BUILDDIR / FLATPAK_REPODIR (set by build_all
+# --ram) redirect the build dir and the ostree repo to tmpfs. The
+# .flatpak-builder cache stays local so source downloads persist between runs.
+BUILDDIR="${FLATPAK_BUILDDIR:-build}"
+REPODIR="${FLATPAK_REPODIR:-repo}"
+flatpak-builder --user --verbose --force-clean --install --repo="$REPODIR" "$BUILDDIR" org.testium.Testium.yaml
 
 # Génère le bundle distribuable
-flatpak build-bundle repo testium.flatpak org.testium.Testium
+flatpak build-bundle "$REPODIR" testium.flatpak org.testium.Testium
 echo "Bundle généré : $(pwd)/testium.flatpak"
 
 # Crée ~/.local/bin/testium pour pouvoir taper "testium" en console