feat(ci): add flatpak build via electron-forge

arm64 currently is broken due to a bug in electron-forge's maker-flatpak, but PR with a fix was submitted already

.github/actions/build-electron/action.yml

@@ -15,10 +15,15 @@ runs:
       if: ${{ inputs.os == 'macos' }}
       shell: bash
       run: brew install python-setuptools
-    - name: Install rpm on Ubuntu for RPM package building
+    - name: Install dependencies for RPM and Flatpak package building
       if: ${{ inputs.os == 'linux' }}
       shell: bash
-      run: sudo apt install rpm
+      run: |
+        sudo apt-get update && sudo apt-get install rpm flatpak-builder elfutils
+        flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+        FLATPAK_ARCH=$(if [[ ${{ inputs.arch }} = 'arm64' ]]; then echo 'aarch64'; else echo 'x86_64'; fi)
+        FLATPAK_VERSION='24.08'
+        flatpak install --user --no-deps --arch $FLATPAK_ARCH --assumeyes runtime/org.freedesktop.Platform/$FLATPAK_ARCH/$FLATPAK_VERSION runtime/org.freedesktop.Sdk/$FLATPAK_ARCH/$FLATPAK_VERSION org.electronjs.Electron2.BaseApp/$FLATPAK_ARCH/$FLATPAK_VERSION
     - name: Install dependencies
       shell: bash
       run: npm ci

.github/workflows/main.yml

@@ -26,7 +26,7 @@ jobs:
             extension: [dmg, zip]
           - name: linux
             image: ubuntu-latest
-            extension: [deb, rpm, zip]
+            extension: [deb, rpm, zip, flatpak]
           - name: windows
             image: windows-latest
             extension: exe
@@ -53,6 +53,7 @@ jobs:
         with:
           name: TriliumNextNotes ${{ matrix.os.name }} ${{ matrix.arch }}.${{matrix.os.extension}}
           path: upload/*.${{ matrix.os.extension }}
+
   build_linux_server:
     name: Build Linux Server
     strategy:

.github/workflows/nightly.yml

@@ -23,7 +23,7 @@ jobs:
             extension: [dmg, zip]
           - name: linux
             image: ubuntu-latest
-            extension: [deb, rpm, zip]
+            extension: [deb, rpm, zip, flatpak]
           - name: windows
             image: windows-latest
             extension: exe
@@ -56,6 +56,7 @@ jobs:
           files: upload/*.*
           tag_name: nightly
           name: Nightly Build
+
   nightly-server:
     name: Deploy server nightly
     strategy:

.github/workflows/release.yml

@@ -23,7 +23,7 @@ jobs:
             extension: [dmg, zip]
           - name: linux
             image: ubuntu-latest
-            extension: [deb, rpm, zip]
+            extension: [deb, rpm, zip, flatpak]
           - name: windows
             image: windows-latest
             extension: exe
@@ -46,6 +46,7 @@ jobs:
           draft: true
           fail_on_unmatched_files: true
           files: upload/*.*
+
   build_linux_server-x64:
     name: Build Linux Server
     strategy: