bug: don't create k8s_downloads_dir if var is not set

[AlisonB319]

What type of PR is this? We were seeing a regression in cse latency. In the discovery the error

+ local k8s_downloads_dir=
...
+ mkdir -p
mkdir: missing operand

was found. Devin's PR - 2ea272b solves the root of the problem, eliminating an unncessary call to extractKubeBinaries, while this PR addresses not using the k8s_downloads_dir if it is not set.

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Special notes for your reviewer:

Release note:

none

[azure-pipelines]

There was an error handling pipeline event d2035ddf-f25b-4dec-89a8-acf304c2fab0.

[Devinwong]

lgtm