flatpak: ci: use .deb flatpak-builder with workaround

When using the flatpak-builder from flatpak, the CI gives error

> dconf-CRITICAL **: 17:56:26.207: unable to create file
> '/run/user/1001/dconf/user': Permission denied.  dconf will not work
> properly.
> [...]
> FB: Running 'flatpak info --arch=x86_64 --show-commit
> org.gnome.Sdk 3.36' on host
> Failed to init: Unable to find sdk org.gnome.Sdk version 3.36

Use flatpak-builder from .deb package, but with file: submodule
workaround.

Change-Id: Icc89cedb4012effd8b69f239382a85fc9ee9517f

.github/workflows/flatpak.yml

@@ -112,8 +112,9 @@ jobs:
           sudo flatpak install --noninteractive flathub \
             org.gnome.Platform//3.36 \
             org.gnome.Sdk//3.36
-          # Use flatpak-builder from flathub; see README.md.
-          sudo flatpak install --noninteractive flathub org.flatpak.Builder
+          # Work around submodule file: problem until .deb package includes
+          # the fix for https://github.com/flatpak/flatpak-builder/issues/495 .
+          git config --global protocol.file.allow always
       - name: Build flatpak package
         run: |
           set -xueo pipefail
@@ -122,7 +123,7 @@ jobs:
           perl -pi -e \
             "s/(\s*).*FW-Auto-Head.*/\$1commit: $(git rev-parse HEAD) # FW-Auto-Head: Generated line. See build script./" \
             org.sil.FieldWorks.yml
-          flatpak run org.flatpak.Builder --verbose --user --sandbox --ccache \
+          flatpak-builder --verbose --user --sandbox --ccache \
             --repo=local-repo --keep-build-dirs --force-clean \
             build-dir org.sil.FieldWorks.yml
       - name: Determine package version