ci: fix checking CI_GITHUB_API_KEY, refactoring (#5496)

* ci: fix checking for presence of CI_GITHUB_API_KEY

CI_GITHUB_API_KEY is always set when evaluating job include/exclude conditions, and it only gets removed when the job starts. Move the check into the script.

* ci: split build jobs into separate shell scripts

tests/ci/style_check.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+#
+# CI job for checking examples style
+
+set -ev
+
+find $TRAVIS_BUILD_DIR/libraries -name '*.ino' -exec \
+    astyle \
+        --suffix=none \
+        --options=$TRAVIS_BUILD_DIR/tests/examples_style.conf {} \;
+
+# Revert changes which astyle might have done to the submodules,
+# as we don't want to fail the build because of the 3rd party libraries
+git submodule foreach --recursive git reset --hard
+
+git diff --exit-code -- $TRAVIS_BUILD_DIR/libraries