Potentially fix problem

nine++ reported that 8ac2eec broke
t/04-nativecall/00-misc.t , which I cannot reproduce.  However,
that commit did change the semantics slightly: instead of an Any,
a Nil would be returned for a non-existing path.  But since the
check used //= , any subsequent attemopt would run the same code
again, as //= does not differentiate between Nil and Any.

This patch does not bind to the loaded hash if the path does not
exist, effectively still having the same semantics as a non-existing
key returns nqp::null.

src/core.c/CompUnit/PrecompilationStore/File.pm6

@@ -202,14 +202,14 @@ class CompUnit::PrecompilationStore::File
             my str $key = $precomp-id.Str;
             nqp::ifnull(
               nqp::atkey($!loaded,$key),
-              nqp::bindkey($!loaded,$key,
-                do {
-                    my $path := self.path($compiler-id, $precomp-id);
-                    $path.e
-                      ?? CompUnit::PrecompilationUnit::File.new(:id($precomp-id), :$path, :store(self))
-                      !! Nil
-                }
-              )
+              do {
+                  my $path := self.path($compiler-id, $precomp-id);
+                  $path.e
+                    ?? nqp::bindkey($!loaded,$key,
+                         CompUnit::PrecompilationUnit::File.new(
+                           :id($precomp-id), :$path, :store(self)))
+                    !! Nil
+              }
             )
         }
     }