CI: Use a libostree with ostreedev/ostree#2592 fixed

We can't use a branch based on libostree git main, because it now needs
a newer GLib than the one in Ubuntu 20.04.

Signed-off-by: Simon McVittie <smcv@collabora.com>

.github/workflows/check.yml

@@ -159,7 +159,7 @@ jobs:
         submodules: true
     - name: Build ostree dependency
       run: |
-        git clone --branch v2022.2 --depth 1 --no-tags https://github.com/ostreedev/ostree.git ./ostree
+        git clone --branch 2022.2-txn-refcount-backport --depth 1 --no-tags https://github.com/smcv/ostree.git ./ostree
         pushd ./ostree
         ./autogen.sh --prefix=/usr --libdir=/usr/lib/x86_64-linux-gnu --sysconfdir=/etc --localstatedir=/var
         make -j $(getconf _NPROCESSORS_ONLN)