fastlane: disable default_keychain to avoid race condition

If we run parallel job and a job sets a temporary keychain into a
default than another job can assume that is the user default.

The result is that one of the jobs will attempt to set a non-existent
temporary keychain into the default keychain.

This happens when we call `Fastlane::Actions::DeleteKeychainAction`:
https://github.com/fastlane/fastlane/blob/2.164.0/fastlane/lib/fastlane/actions/delete_keychain.rb#L21

Signed-off-by: Jakub Sokołowski <jakub@status.im>

fastlane/Fastfile

@@ -91,8 +91,8 @@ class Keychain
       unlock: true,
       # Lock it up after 25 minutes just in case we don't delete it.
       timeout: 1500,
-      # To make Fastlane import WWDR certificate into it.
-      default_keychain: true,
+      # Setting a default can cause a race condition with parallel jobs.
+      default_keychain: false,
     )
   end