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.
9fcd3bffc2