CI: Install 'serde-yaml 0.9' from experimental

To make it possible for the CI to pass, during the transition
from 'serde-yaml 0.8' to 'serde-yaml 0.9' in Debian Trixie.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/624>

.gitlab-ci.yml

@@ -23,6 +23,7 @@ stages:
   - deploy
 
 before_script:
+  - echo "deb http://deb.debian.org/debian/ experimental main" > /etc/apt/sources.list.d/experimental.list
   - apt-get -y update
   - apt-get -y install ca-certificates
 
@@ -45,6 +46,7 @@ build_meson:
       - _build
     expire_in: 3h
   script:
+    - apt-get -y install -t experimental librust-serde-yaml-dev
     - apt-get -y build-dep .
     - meson . _build/ -Ddepdatadir=/usr/share -Dfind_orphans=true --werror
     - ninja -C _build install
@@ -61,6 +63,7 @@ build_deb:
       - '*.deb'
   script:
     - rm -f ../*.deb
+    - apt-get -y install -t experimental librust-serde-yaml-dev
     - apt-get -y build-dep .
     - apt-get -y install devscripts
     - REV=$(git log -1 --format=%h)
@@ -80,6 +83,7 @@ build_reference:
     paths:
       - _build/doc
   script:
+    - apt-get -y install -t experimental librust-serde-yaml-dev
     - apt-get -y build-dep .
     - apt-get -y install cargo
     - cd _build
@@ -94,6 +98,7 @@ test:
     - job: build_meson
       artifacts: true
   script:
+    - apt-get -y install -t experimental librust-serde-yaml-dev
     - apt-get -y build-dep .
     - apt-get -y install clang-tidy
     - ninja -C _build test
@@ -106,6 +111,7 @@ test_style:
   stage: test
   needs: []
   script:
+    - apt-get -y install -t experimental librust-serde-yaml-dev
     - apt-get -y build-dep .
     - tools/style-check_source
   except: