GH Action/update-images: escape back-quote when echoing PR body

Make sure that shell does not interpret the text within the back-quote
as a command to execute in a sub-shell.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

.github/workflows/update-images.yml

@@ -42,7 +42,7 @@
               git add Schutzfile
               git commit -m "Schutzfile: Update images dependency ref to latest"
               git push -f https://"$GITHUB_TOKEN"@github.com/schutzbot/osbuild.git
-              echo "Updating images dependency ref to current `main`" > body
+              echo "Updating images dependency ref to current \`main\` > body
               gh pr create \
                 -t "Update images dependency ref to latest" \
                 -F "body" \