[iOS] Handle double taps on the passkey welcome screen primary button

The primary action button of the passkey welcome screen (especially the
one leading to the enrollment process), can be tapped multiple times
before the action is executed and the next screen is presented. On the
second tap, `_primaryButtonAction` is `nil` resulting in a crash.

Another (an probably more common) scenario where the primary button
can be tapped more than once is when the enrollment keychain UI (shown
when tapping the primary button the first time) is dismissed with a
down swipe, leaving the passkey welcome screen at the top of the
navigation stack again. The button then becomes available an can be
tapped a second time, but crashes as `_primaryButtonAction` has become
`nil`.

This CL fixes the issue by only executing the block if the current
passkey welcome screen is still at the top of the navigation stack
(i.e., additional taps, if any, are ignored), and by allowing
`_primaryButtonAction` to be executed more than once by not setting it
to `nil` after it's first used.

Fixed: 379172052
Change-Id: I5df4f1769330dcc4b0568eab880f89a8780831e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6037963
Commit-Queue: Noémie St-Onge <noemies@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385853}