Shellcheck quoting (#9596)

* Fix SC2006: Use $(...) notation instead of legacy backticked `...`.

* Fix SC2016: Expressions don't expand in single quotes, use double quotes for that.

* Fix SC2046: Quote this to prevent word splitting.

* Fix SC2053: Quote the right-hand side of == in [[ ]] to prevent glob matching.

* Fix SC2068: Double quote array expansions to avoid re-splitting elements.

* Fix SC2086: Double quote to prevent globbing and word splitting.

* Fix SC2102: Ranges can only match single chars (mentioned due to duplicates).

* Fix SC2140: Word is of the form "A"B"C" (B indicated). Did you mean "ABC" or "A\"B\"C"?

* Fix SC2145: Argument mixes string and array. Use * or separate argument.

* Fix SC2209: warning: Use var=$(command) to assign output (or quote to assign string).

Co-authored-by: Mehrdad <noreply@github.com>

python/ray/autoscaler/kubernetes/kubectl-rsync.sh

@@ -6,7 +6,7 @@
 
 if [ -z "$KRSYNC_STARTED" ]; then
     export KRSYNC_STARTED=true
-    exec rsync --blocking-io --rsh "$0" $@
+    exec rsync --blocking-io --rsh "$0" "$@"
 fi
 
 # Running as --rsh
@@ -22,4 +22,4 @@ if [ "X$pod" = "X-l" ]; then
     shift
 fi
 
-exec kubectl $namespace exec -i $pod -- "$@"
+exec kubectl "$namespace" exec -i "$pod" -- "$@"