Fix editable-install patch for 26.05 + unblock icon in unprivileged build

The previous patch matched a development-branch variable that does not
exist in release 2605.0, so the warning fired and the editable install was
not patched. In 2605.0 the -e is hardcoded in the pip command
("pip install -e ${package_folder_path}"); strip the -e there instead.

Also, flatpak build-export validates exported app icons in a bwrap sandbox,
which fails in an unprivileged container. Keep the icon inside the app (for
the running window/taskbar) but remove it from build-dir/export so export
skips validation. Host menu icon stays generic; build needs no privileges.

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

README.md

@@ -133,17 +133,21 @@ These were confirmed by inspecting the v26.05 package (`opt/O3DE/26.05/…`):
   home path, so it can only be created at runtime.
 - **The editable-install patch.** O3DE installs its own `o3de` CLI with
   `pip install -e`, which writes an `egg-info` next to the source under read-only
-  `/app` and fails. The build patches `cmake/LYPython.cmake` to use a normal
-  (non-editable) install, which builds in a temp dir and lands in the `~/.o3de`
-  venv. If a future O3DE release renames that variable, the build prints a warning
-  and Python setup will fail at runtime — that's the line to re-check.
+  `/app` and fails. The build patches `cmake/LYPython.cmake` to drop the `-e`, so
+  it's a normal install (built in a temp dir, landing in the writable `~/.o3de`
+  venv). If a future O3DE release changes that pip command, the build prints a
+  warning and Python setup will fail at runtime — that's the line to re-check.
 - **Other runtime writes into the install tree may still fail.** Anything else
   O3DE tries to generate *inside* `/opt/O3DE/...` at runtime (e.g. certain per-gem
   Python deps, or the engine-level asset cache) hits the read-only `/app`. Project
   data lives in your writable home dir, so normal project work should be fine;
   this is the main remaining open risk.
-- **Launcher icon** is shipped as `org.o3de.O3DE.png` (the `.deb` itself contains
-  only in-editor asset icons, none suitable as an app icon).
+- **Launcher icon.** Shipped as `org.o3de.O3DE.png` (the `.deb` itself has only
+  in-editor asset icons). It's kept *inside* the app so the running window/taskbar
+  shows it, but removed from the *exported* set because `flatpak build-export`
+  validates exported icons in a bwrap sandbox that fails in an unprivileged
+  container. Consequence: the host menu launcher icon is generic. A privileged
+  runner would let us export it properly.
 - **GPU / drivers:** the renderer needs working GPU access. The manifest grants
   `--device=dri`/`--device=all`; on some setups you may also want the matching
   GPU driver extension from Flathub.