Update gem signing

This way it generates fewer warnings.

ttfunk.gemspec

@@ -12,14 +12,14 @@ Gem::Specification.new do |spec|
     'rubygems_mfa_required' => 'true',
   }
 
-  signing_key = File.expand_path('~/.gem/gem-private_key.pem')
-  if File.exist?(signing_key)
-    spec.cert_chain = ['certs/pointlessone.pem']
-    if $PROGRAM_NAME.end_with?('gem')
+  if File.basename($PROGRAM_NAME) == 'gem' && ARGV.include?('build')
+    signing_key = File.expand_path('~/.gem/gem-private_key.pem')
+    if File.exist?(signing_key)
+      spec.cert_chain = ['certs/pointlessone.pem']
       spec.signing_key = signing_key
+    else
+      warn 'WARNING: Signing key is missing. The gem is not signed and its authenticity can not be verified.'
     end
-  else
-    warn 'WARNING: Signing key is missing. The gem is not signed and its authenticity can not be verified.'
   end
 
   spec.authors = [