ci: Add shellcheck for scripts (#20631)

Fixes shellcheck errors in script/*
Adds a couple trailing newlines.
Adds `script/shellcheck-scripts` and associated CI machinery.
Current set ultra-conservative, does not output warnings, only errors.

script/clear-target-dir-if-larger-than

@@ -2,7 +2,7 @@
 
 set -eu
 
-if [[ $# < 1 ]]; then
+if [[ $# -ne 1 ]]; then
     echo "usage: $0 <MAX_SIZE_IN_GB>"
     exit 1
 fi