Don't try to load precomp files after changing the precomp store

In the following sequence:
use Test;
use lib 't/spec';
use Test::Utils;
after the 'use lib' we not only may no longer precompile modules, but also may
not load precompiled modules. That's because the precompiled Test::Utils may
link to a different precompiled version of Test. On loading Test::Utils this
Test module would be loaded and conflict with the already loaded Test.

The moral of the story is: to take most advantage of precompilation, try to set
up your repository chain before starting to load modules.

src/core/CompUnit/Repository/FileSystem.pm

@@ -22,7 +22,7 @@ class CompUnit::Repository::FileSystem does CompUnit::Repository::Locally does C
         my $dir-sep           := $*SPEC.dir-sep;
         my $name               = $spec.short-name;
         my $id = nqp::sha1($name ~ $*REPO.id);
-        my $handle = $precomp.load($id);
+        my $handle = $precomp.may-precomp ?? $precomp.load($id) !! CompUnit::Handle;
         my $base := $!prefix.abspath ~ $dir-sep ~ $name.subst(:g, "::", $dir-sep) ~ '.';
         my $compunit;