[ci] Add shellcheck linting to CI

Signed-off-by: Miles Dai <milesdai@google.com>
diff --git a/apt-requirements.txt b/apt-requirements.txt
index 62dd475..36b110c 100644
--- a/apt-requirements.txt
+++ b/apt-requirements.txt
@@ -44,6 +44,7 @@
 python3-setuptools
 python3-urllib3
 python3-wheel
+shellcheck
 srecord
 tree
 xsltproc