Change _async to default True

Change _async to default True instead of False to mimic previous behavior.

Signed-off-by: ttrently <41705925+ttrently@users.noreply.github.com>

src/rez/package_cache.py

@@ -366,7 +366,7 @@ def remove_variant(self, variant):
 
         return self.VARIANT_REMOVED
 
-    def add_variants_async(self, variants, _async=False):
+    def add_variants_async(self, variants, _async=True):
         """Update the package cache by adding some or all of the given variants.
 
         This method is called when a context is created or sourced. Variants
@@ -461,6 +461,8 @@ def add_variants_async(self, variants, _async=False):
                     out_target = devnull
 
                 func = subprocess.Popen
+
+                # use subprocess.call if we are not running async since it is blocking
                 if not _async:
                     func = subprocess.call