buildEnv: improve file check to avoid false-positives

The original change in #55372 was supposed to fix the case where a store
path which is a file should be placed into `buildEnv` which broke with a
fairly misleading Perl error.

Unfortunately this introduced a regression, `findFiles` can have targets
that are files if the file isn't a store path. Rather than adding more
obscure checks with probably further regressions, I figured that it's
better to replicate the behavior of `lib.isStorePath` and explicitly
check if the store path is a file and break in this case only.

This should also fix recent staging issues.

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