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/builder.pl

@@ -26,6 +26,13 @@ sub isInPathsToLink {
     return 0;
 }
 
+# Similar to `lib.isStorePath`
+sub isStorePath {
+    my $path = shift;
+    my $storePath = "@storeDir@";
+
+    return substr($path, 0, 1) eq "/" && dirname($path) eq $storePath;
+}
 
 # For each activated package, determine what symlinks to create.
 
@@ -84,8 +91,9 @@ sub checkCollision {
 sub findFiles {
     my ($relName, $target, $baseName, $ignoreCollisions, $checkCollisionContents, $priority) = @_;
 
-    if (-f $target) {
-        die "Path $target is a file and can't be merged into an environment using pkgs.buildEnv!";
+    # The store path must not be a file
+    if (-f $target && isStorePath $target) {
+        die "The store path $target is a file and can't be merged into an environment using pkgs.buildEnv!";
     }
 
     # Urgh, hacky...