try to unflakify actions/runner-images#675

Author: ladyada

actions_install.sh

@@ -3,8 +3,15 @@
 set -e
 
 pip3 install clint pyserial setuptools adafruit-nrfutil
-sudo apt-get update
-sudo apt-get install -y libllvm8 -V
+sudo gem install apt-spy2
+sudo apt-spy2 check
+sudo apt-spy2 fix --commit
+
+# after selecting a specific mirror, we need to run 'apt-get update'
+sudo apt-get -o Acquire::Retries=3 update
+
+sudo apt-get -o Acquire::Retries=3 install -y libllvm8 -V
+
 sudo apt install -fy cppcheck clang-format-8
 if [ ! -f /usr/bin/clang-format ]; then
     sudo ln -s /usr/bin/clang-format-8 /usr/bin/clang-format