fix: Link test race condition (#6081)
### Description
We have a flakey link test where it reads the environment variables for
`ClientConfig` and occasionally errors if `REMOTE_CACHE_TIMEOUT` is set
to an empty string. Instead of reading the environment, we now provide
an empty map.
### Testing Instructions
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Closes TURBO-1394
Co-authored-by: nicholaslyang <Nicholas Yang>
