fix(appimage): build AppRun with bounded argument splitting (#16330)

* fix(appimage): build AppRun with bounded argument splitting

AppRun v2.0.0, which appimage-builder downloads, copies each argument
into a 1 KiB stack buffer in apprun_shell_split_arguments, so any
argument of 1024 bytes or more aborts the AppImage with "buffer overflow
detected" before RustDesk starts (e.g. --connect with a 10000-char id).

Build AppRun from the v2.0.0 commit with a patch that sizes the buffer
from the input and stops the quote/escape scanning at the terminating
NUL, in ubuntu:16.04 like upstream's release, and put it where
appimage-builder looks before downloading.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* fix(appimage): verify the patched AppRun is the one shipped

Reset the split buffer by its first byte rather than zeroing all of it
per argument, check the AppRun source tarball's sha256, pin the build
image by digest, and fail the job if appimage-builder deployed anything
other than the binary built here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/flutter-build.yml

@@ -2444,7 +2444,9 @@ jobs:
           sudo pip3 install git+https://github.com/rustdesk-org/appimage-builder.git
           # run appimage-builder
           pushd appimage
+          ./build-apprun.sh ${{ matrix.job.arch }}
           sudo appimage-builder --skip-tests --recipe ./AppImageBuilder-${{ matrix.job.arch }}.yml
+          cmp AppDir/AppRun appimage-build/AppRun/v2.0.0/AppRun-Release-${{ matrix.job.arch }}
 
       - name: Publish appimage package
         if: env.UPLOAD_ARTIFACT == 'true'