Fix loaded-symbol check for the staging-repo class (#617)

(try ::($name)) was always False for any loaded class: ::($name)
returns the class's type object when bound, type objects coerce to
False in Boolean context, and the try wraps a missing-symbol Failure
into Any (also False). So the `?? ::($name).new ...` branch never
ran; the `!! do { ... $core-repo.resolve(...).distribution ... }`
fallback took every call.

Drop the try. Bare ::($name) returns the type object when the symbol
is bound and a Failure when it is not, so `!~~ Failure` distinguishes
them and the fast path can be reached when the class is already
loaded (which is always the case once CompUnit::Repository::Staging
moves into CORE.setting).

Author: ugexe

lib/Zef/Client.rakumod

@@ -1076,7 +1076,7 @@ class Zef::Client {
             # for instance).
             my %curfs-new-args = :prefix($staging-at), :name($stage-for-repo.name), :next-repo($stage-for-repo);
             my $curfs-short-name = 'CompUnit::Repository::Staging';
-            (try ::($curfs-short-name))
+            (::($curfs-short-name) !~~ Failure)
                 ?? ::($curfs-short-name).new(|%curfs-new-args)
                 !! do {
                     # Find CURS from the core repository so we can load it by path later