Fix upload-only to retrieve the METRICS_UPLOADER variable

in a timely manner.

There should not be harm in bypassing fetchEnvConfig for this case
as the metrics uploader should not be present for invalid cases.

Bug: b/264905338
Test: b build libcore:all
Test: printfs to verify that upload.go has the variable set.
Change-Id: Ia7d03f25e74d4ec2d6cb83793b793a23b47f26de