build: update browserstack key

[josephperrott]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit

PR Type

What kind of change does this PR introduce?
Update BrowserStack key.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

BrowserStack fails.

What is the new behavior?

BrowserStack works!

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing
     status "code-review/pullapprove" is pending

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[angular-automatic-lock-bot]

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