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.

pkgs/build-support/buildenv/default.nix

@@ -2,7 +2,7 @@
 # a fork of the buildEnv in the Nix distribution.  Most changes should
 # eventually be merged back into the Nix distribution.
 
-{ buildPackages, runCommand, lib }:
+{ buildPackages, runCommand, lib, substituteAll }:
 
 lib.makeOverridable
 ({ name
@@ -43,6 +43,13 @@ lib.makeOverridable
 , meta ? {}
 }:
 
+let
+  builder = substituteAll {
+    src = ./builder.pl;
+    inherit (builtins) storeDir;
+  };
+in
+
 runCommand name
   rec {
     inherit manifest ignoreCollisions checkCollisionContents passthru
@@ -67,6 +74,6 @@ runCommand name
     passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else null;
   }
   ''
-    ${buildPackages.perl}/bin/perl -w ${./builder.pl}
+    ${buildPackages.perl}/bin/perl -w ${builder}
     eval "$postBuild"
   '')