Use let to initialise key to avoid reusing same key name on retry

What does this MR do and why?

Fixes: #370107 (closed) #327651 (closed)

Currently if this spec fails for some reason, when it retries the test it will reuse the same key_title. While this may allow the 'user can add an SSH key' spec to pass - it them causes failures in can delete an ssh key

Ambiguous match, found 2 elements matching visible link "E2E test key: key for ssh tests 1660046708.1086028"

We should instead use let to create this key_title so if the test fails, we can rerun it with a new name to avoid the ambiguous errors.

How to set up and validate locally

We need to force the spec to fail the first time it runs - I found it easiest to test this by manually adding a sleep, and then when the test was waiting, I manually navigated away to a random page http://gdk.test/abc/def so the assertion failed the first time. I then let it run as normal for the 'retry'.

      it 'user can add an SSH key', testcase: 'https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/347819' do
        key = Resource::SSHKey.fabricate_via_browser_ui! do |resource|
          resource.title = key_title
        end
        sleep 10
        expect(page).to have_content(key.title)
        expect(page).to have_content(key.sha256_fingerprint)
      end

1. export CI=true (to allow rerunning tests)
2. Run the test with export WEBDRIVER_HEADLESS=false
3. Run with bundle exec bin/qa Test::Instance::All http://gdk.test:3000 -- qa/specs/features/browser_ui/3_create/repository/ssh_key_support_spec.rb
4. When the test fails, it should retry - note prior to this change the key title is reused causing Ambiguous errors. With this change a new key title is used for the retry attempt.

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