.github: Add a script for figuring out a branch from channel name

It will be used for deduplicating the github workflows.

sdk_container/src/third_party/coreos-overlay/.github/workflows/figure-out-branch.sh

@@ -0,0 +1,71 @@
+#!/bin/bash
+
+# Prints the following github outputs based on channel named passed to
+# the script as a parameter.
+#
+# BRANCH is a name of the git branch related to the passed channel.
+#
+# SKIP tells whether the rest of the steps should be skipped, will be
+# either 0 or 1.
+#
+# LINK is a link to release mirror for the following channel. Will be
+# empty for main channel.
+#
+# LABEL is going to be mostly the same as the channel name, except
+# that lts-old will be labeled as lts.
+
+set -euo pipefail
+
+if [[ ${#} -ne 1 ]]; then
+    echo "Expected a channel name as a parameter" >&2
+    exit 1
+fi
+
+channel_name="${1}"
+skip=0
+link=''
+branch=''
+label=''
+case "${channel_name}" in
+    main)
+        branch='main'
+        ;;
+    lts-old)
+        curl -fsSLO --retry-delay 1 --retry 60 --retry-connrefused --retry-max-time 60 --connect-timeout 20 'https://lts.release.flatcar-linux.net/lts-info'
+        if [[ $(grep -e ':supported' lts-info | wc -l) -le 1 ]]; then
+            # Only one supported LTS, skip this workflow run
+            # as 'lts' matrix branch will handle updating the only
+            # supported LTS.
+            skip=1
+        else
+            line=$(grep -e ':supported' lts-info | sort -V | head -n 1)
+            major=$(awk -F: '{print $1}' <<<"${line}")
+            year=$(awk -F: '{print $2}' <<<"${line}")
+            branch="flatcar-${major}"
+            # Drop this corner case when 2605 is not supported.
+            if [[ ${major} -eq 2605 ]]; then
+                branch='flatcar-lts-2605'
+            fi
+            link="https://lts.release.flatcar-linux.net/amd64-usr/current-${year}"
+            label='lts'
+        fi
+        rm -f lts-info
+        ;;
+    alpha|beta|stable|lts)
+        link="https://${channel_name}.release.flatcar-linux.net/amd64-usr/current"
+        major=$(curl -sSL "${link}/version.txt" | awk -F= '/FLATCAR_BUILD=/{ print $2 }')
+        branch="flatcar-${major}"
+        ;;
+    *)
+        echo "Unknown channel '${channel_name}'" >&2
+        exit 1
+esac
+
+if [[ -z "${label}" ]]; then
+    label="${channel_name}"
+fi
+
+echo "BRANCH=${branch}" >>"${GITHUB_OUTPUT}"
+echo "SKIP=${skip}" >>"${GITHUB_OUTPUT}"
+echo "LINK=${link}" >>"${GITHUB_OUTPUT}"
+echo "LABEL=${label}" >>"${GITHUB_OUTPUT}"