project: Only check non-cache vars when setting project vars

The change in commit 86ad7cc886 (project: Only define non-cache vars if
already defined, 2024-09-15, v3.30.4~2^2) was meant to only check for
non-cache variables when deciding whether to set non-cache project
variables for the current call.  However, it erroneously checked for any
variable, including cache variables.  This gives the intended result on
the first run, but on subsequent runs a cache variable will exist that
did not on the first run, leading to different behavior between the two
runs.  Fix the logic to only check for a pre-existing non-cache
variable, as was originally intended.

Fixes: #26355

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