Fix a missing pre-comp store unlock.

This could cause the store to remain locked until program exit. This
was particularly problematic on Windows parallel spectests, causing a
hang. The test harness seems to read from the processes one at a time
on Windows, blocking the output of those it's not currently reading
from. A test that needed to pre-compile something while running would
block on acquiring the lock, which was being held by another script that
would not release it until process exit. In combination, this created a
circular wait, and thus a deadlock. However, there are probably platform
independent deadlocks that could have been caused by this bug, not to
mention inefficiency.

src/core/CompUnit/PrecompilationRepository.pm

@@ -89,7 +89,13 @@ class CompUnit::PrecompilationRepository::Default does CompUnit::PrecompilationR
         if $path {
             my $modified = $path.modified;
             if (not $since or $modified > $since) and self!load-dependencies($path, $modified) {
-                %!loaded{$id} //= self!load-handle-for-path($path)
+                with %!loaded{$id} {
+                    self.store.unlock;
+                    $_
+                }
+                else {
+                    %!loaded{$id} = self!load-handle-for-path($path)
+                }
             }
             else {
                 if $*RAKUDO_MODULE_DEBUG -> $RMD {