devops: fix Python installation for Firefox Beta build on Ubuntu 18.04 (#16152)

browser_patches/docker_build.sh

@@ -146,7 +146,7 @@ function ensure_docker_container {
     # Firefox build on Ubuntu 18.04 requires Python3.8 to run its build scripts.
     # WebKit build on Ubuntu 18.04 fails with the Python 3.8 installation but works
     # with Python 3.6 that is shipped as default python3 on Ubuntu 18.
-    if [[ "${BUILD_FLAVOR}" == "firefox-ubuntu-18.04" ]]; then
+    if [[ "${BUILD_FLAVOR}" == "firefox-ubuntu-18.04" || "${BUILD_FLAVOR}" == "firefox-beta-ubuntu-18.04" ]]; then
       apt-get install -y python3.8 python3.8-dev python3.8-distutils
       # Point python3 to python3.8
       update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2