Run "core update-index" before "core install"

Running

    $HOME/arduino-cli --format json core install adafruit:avr --additional-urls https://adafruit.github.io/arduino-board-index/package_adafruit_index.json

for the first time fails with error message

    Error initializing instance: Loading index file: loading json index file $HOME/.arduino15/package_adafruit_index.json: open $HOME/.arduino15/package_adafruit_index.json: no such file or directory
    Error initializing instance: Loading index file: loading json index file $HOME/.arduino15/package_adafruit_index.json: open $HOME/.arduino15/package_adafruit_index.json: no such file or directory
    Invalid argument passed: Found 0 platform for reference "adafruit:avr":

However by running an update-index command first, then it succeeds, e.g.

    $HOME/arduino-cli --format json core update-index --additional-urls https://adafruit.github.io/arduino-board-index/package_adafruit_index.json
    $HOME/arduino-cli --format json core install adafruit:avr --additional-urls https://adafruit.github.io/arduino-board-index/package_adafruit_index.json

https://arduino.github.io/arduino-cli/0.29/getting-started/#adding-3rd-party-cores

Author: hlovdal

Diff for: lib/arduino_ci/arduino_backend.rb

@@ -182,7 +182,10 @@ def install_boards(boardfamily)
       result = if @additional_urls.empty?
         run_and_capture("core", "install", boardfamily)
       else
-        run_and_capture("core", "install", boardfamily, "--additional-urls", @additional_urls.join(","))
+        urls = @additional_urls.join(",")
+        res1 = run_and_capture("core", "update-index", "--additional-urls", urls)
+        res2 = run_and_capture("core", "install", boardfamily, "--additional-urls", urls)
+        Host.merge_capture_results([res1, res2])
       end
       result[:success]
     end