Mark Nexus password as a sensitive field

[liamjones]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass - these were failing before I made my changes (may be my setup). bundle exec rspec ./fastlane/spec/actions_specs/nexus_upload_spec.rb worked
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've read the Contribution Guidelines
• I've updated the documentation if necessary. - N/A
• I've added or updated relevant unit tests. - N/A (I think)

Motivation and Context

When prompted for the Nexus password, nexus_upload was printing out what the user entered.

Description

Marked field as sensitive.

Testing Steps

Ran bundle exec fastlane run nexus_upload to confirm asterisks printed rather than the password after the change.
Existing tests for nexus_upload still pass, I can't see how I'd test the addition of the sensitive flag.

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[getaaron]

Thanks!