Add constructor option fixed_paths

The name indicates that it will remain the same despite changes to
`$ENV{PATH}`.

lib/File/Which.pm

@@ -131,6 +131,10 @@ sub new {
 
   $self->{PATHEXT} = $self->_default_pathext;
 
+  if( exists $opts{fixed_paths} ) {
+    $self->{fixed_paths} = $opts{fixed_paths};
+  }
+
   $self;
 }
 
@@ -249,7 +253,9 @@ sub which {
   return $exec  ## no critic (ValuesAndExpressions::ProhibitMixedBooleanOperators)
           if !$self->_is_vms and !$self->_is_mac and !$self->_is_win and $exec =~ /\// and -f $exec and -x $exec;
 
-  my @path = @{ $self->_default_path };
+  my @path = exists $self->{fixed_paths}
+    ? @{ $self->{fixed_paths} }
+    : @{ $self->_default_path };
 
   my @PATHEXT = @{ $self->{PATHEXT} };
   foreach my $base ( map { File::Spec->catfile($_, $exec) } @path ) {