ci: Add job for pushing coverity reports

This job does:
1. Download the coverity bundle and untar it in a cached location
2. Build GNOME Shell using clang and the coverity tool
3. Compress the coverity report
4. Upload for analysis

In a similar setup to that of Mutter.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1913>

.gitlab-ci/download-coverity-tarball.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/bash
+
+# We need a coverity token to fetch the tarball
+if [ -x $COVERITY_TOKEN ]
+then
+  echo "No coverity token. Run this job from a protected branch."
+  exit -1
+fi
+
+mkdir -p coverity
+
+# Download and check MD5 first
+curl https://scan.coverity.com/download/linux64 \
+  --data "token=$COVERITY_TOKEN&project=GNOME+Shell&md5=1" \
+  --output /tmp/coverity_tool.md5
+
+diff /tmp/coverity_tool.md5 coverity/coverity_tool.md5 >/dev/null 2>&1
+
+if [ $? -eq 0 -a -d coverity/cov-analysis* ]
+then
+  echo "Coverity tarball is up-to-date"
+  exit 0
+fi
+
+# Download and extract coverity tarball
+curl https://scan.coverity.com/download/linux64 \
+  --data "token=$COVERITY_TOKEN&project=GNOME+Shell" \
+  --output /tmp/coverity_tool.tgz
+
+rm -rf ./coverity/cov-analysis*
+
+tar zxf /tmp/coverity_tool.tgz -C coverity/
+if [ $? -eq 0 ]
+then
+  mv /tmp/coverity_tool.md5 coverity/
+fi
+
+rm /tmp/coverity_tool.tgz