Update gitlab-ci pipewire to version 0.3.31

Updating PipeWire to the latest version lets us check that everything
still works well, and shows users that they are able to use it.

[daniels: Updated to 0.3.31, use symbolic ref tags rather than SHAs.
          0.3.32 is released, but doesn't build in our Debian
          environment; this is fixed upstream but there is no release
	  for it yet.]

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

.gitlab-ci.yml

@@ -20,7 +20,7 @@ stages:
   variables:
     FDO_DISTRIBUTION_VERSION: buster
     FDO_DISTRIBUTION_EXEC: 'bash .gitlab-ci/debian-install.sh'
-    FDO_DISTRIBUTION_TAG: '2021-04-15.2'
+    FDO_DISTRIBUTION_TAG: '2021-07-31.0-pipewire'
 
 
 container_prep:

.gitlab-ci/debian-install.sh

@@ -154,9 +154,8 @@ cd ..
 rm -rf mesa
 
 rm -rf pipewire
-git clone --single-branch --branch master https://gitlab.freedesktop.org/pipewire/pipewire.git pipewire
+git clone --depth=1 --branch 0.3.31 https://gitlab.freedesktop.org/pipewire/pipewire.git pipewire
 cd pipewire
-git checkout -b snapshot db12f47505ddccb257acdc0fa2bb884aceb1b593
 meson build
 ninja -C build install
 cd ..