Only install clang-format if it is really missing

As the default image already has v10, v11 (default) and v12 installed,
you don't have to install v8 and don't even use it. This removes the
need for a long (most of the time 2 minutes) mirror search or even call
'apt-get install' in the first place. This shortens the time spent in
actions_install.sh from about 2:30min to 6s, which is 25x faster.

Author: eringerli

actions_install.sh

@@ -3,17 +3,20 @@
 set -e
 
 pip3 install clint pyserial setuptools adafruit-nrfutil
-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
+# Only install stuff if it is really missing. This should never be executed,
+# as the default image contains clang-format v10, v11 (default) and v12.
+# https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#language-and-runtime
+if [ ! -f /usr/bin/clang-format ]; then
+    sudo gem install apt-spy2
+    sudo apt-spy2 check
+    sudo apt-spy2 fix --commit
 
-sudo apt-get -o Acquire::Retries=3 install -y libllvm8 -V
+    # 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 clang-format-8 libllvm8
 
-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
 fi