Build: Add gh action to update homebrew

this patch adds a gh action to create PR
which updatest headlamp version and
sha of assets for homebrew
Signed-off-by: Santhosh Nagaraj S <santhosh@kinvolk.io>

.github/workflows/pr-to-update-homebrew.yml

+name: PR to update homebrew
+
+# This action will run after a tag starting with "v" is published
+on:
+  push:
+    tags:
+    - 'v*'
+  workflow_dispatch:
+
+env:
+  LATEST_HEADLAMP_TAG: latest
+jobs:
+  create_pr_to_upgrade_homebrew:
+    name: Create PR to upgrade homebrew
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout headlamp repo 
+        uses: actions/checkout@v2 
+        with:
+          path: headlamp
+          fetch-depth: 0
+      - name: Configure Git 
+        run: | 
+          user=${{github.actor}}
+          if [ -z $user ]; then
+            user=yolossn
+          fi
+          git config --global user.name "$user"
+          git config --global user.email "$user@users.noreply.github.com"
+      - name: Get headlamp latest tag 
+        run: |
+          cd headlamp 
+          latestTag=$(git tag --list --sort=version:refname 'v*' | tail -1)
+          echo "LATEST_HEADLAMP_TAG=$latestTag" >> $GITHUB_ENV
+          echo $latestTag
+      - name: Sync homebrew-cask fork from upstream
+        run: |
+          gh repo sync kinvolk/homebrew-cask
+        env:
+          GITHUB_TOKEN: ${{ secrets. KINVOLK_REPOS_TOKEN }}
+      - name: Check out homebrew-cask repo 
+        uses: actions/checkout@v2 
+        with: 
+          repository: kinvolk/homebrew-cask
+          path: homebrew-cask
+          token: ${{ secrets. KINVOLK_REPOS_TOKEN }}
+      - name: Update headlamp version in homebrew-cask
+        run: |
+          user=${{github.actor}}
+          if [ -z $user ]; then
+            user=yolossn
+          fi
+          cd homebrew-cask
+          HEADLAMP_VERSION=${LATEST_HEADLAMP_TAG:1}
+          wget "https://github.com/kinvolk/headlamp/releases/download/$LATEST_HEADLAMP_TAG/checksums.txt"
+          ARM_SHA=$(cat checksums.txt | grep .arm64.dmg | awk -F" " '{print $1}')
+          INTEL_SHA=$(cat checksums.txt | grep .x64.dmg | awk -F" " '{print $1}')
+          git checkout -b "update_headlamp_$HEADLAMP_VERSION"
+          sed -i "s/version\ .*/version \"$HEADLAMP_VERSION\"/g" ./Casks/headlamp.rb
+          if [ $ARM_SHA ]; then
+            echo "replacing ARM SHA"
+            sed -i "s/sha256 arm:\   \".*/sha256 arm:\   \"$ARM_SHA\",/g" ./Casks/headlamp.rb
+          fi
+          if [ $INTEL_SHA ]; then
+            echo "replacing Intel SHA"
+            sed -i "s/  intel:\ \".*/  intel:\ \"$INTEL_SHA\"/g" ./Casks/headlamp.rb
+          fi
+          git diff
+          rm ./checksums.txt
+          git add ./Casks/headlamp.rb
+          git status 
+          git commit --signoff -m "Update Headlamp version to $HEADLAMP_VERSION"
+          git status 
+          git log -1 
+          git push origin "update_headlamp_$HEADLAMP_VERSION" -f
+          gh pr create \
+          --title "Upgrade Headlamp version to $HEADLAMP_VERSION" \
+          --repo "Homebrew/homebrew-cask" \
+          --head "kinvolk:update_headlamp_$HEADLAMP_VERSION" \
+          --base "master"  \
+          --assignee "$user" \
+          --body "Upgrade Headlamp version to $HEADLAMP_VERSION
+            cc: @$user" \
+        env:
+          LATEST_HEADLAMP_TAG: ${{ env.LATEST_HEADLAMP_TAG }}
+          GITHUB_TOKEN: ${{ secrets. KINVOLK_REPOS_TOKEN }}