[action][spm] deprecate build_path option in favor of scratch_path, as recommended by Swift CLI

[Tavernari]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• 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.

Motivation and Context

The swift cli has been warning about the build-path deprecation.

Description

This PR adds the option to use scratch_path instead of build_path, but it kept the build_path available since the swift cli is still accepting it as a warning.
I tested it, just changing the option.

Testing Steps

Using this PR as the Fastlane source.

1. Create an SPM project
2. Initialise fastlane
3. Add to their Fastfile the spm action with scratch_path instead of build_path
4. Change scratch_path to build_path and it should work as well.

[google-cla]

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[Tavernari]

Failed examples:

rspec ./fastlane/spec/actions_specs/update_fastlane_spec.rb:13 # Fastlane Fastlane::FastFile update_fastlane when no update needed
rspec ./fastlane/spec/actions_specs/update_fastlane_spec.rb:28 # Fastlane Fastlane::FastFile update_fastlane when update with RubyGems
rspec ./fastlane/spec/actions_specs/update_fastlane_spec.rb:68 # Fastlane Fastlane::FastFile update_fastlane when update with Homebrew

I am getting these errors when running bundle exec rspec.

[joshdholtz]

Didn't know about thisscratch-path change 😅 Gave one suggestion and then we can get this merged!

[Tavernari]

Didn't know about thisscratch-path change 😅 Gave one suggestion and then we can get this merged!

What kind of suggestion?

[lacostej]

I think @joshdholtz's requested changes have been accepted. So we should be good to merge this.

Thanks for the contribution!

[rogerluan]

💯 Neat!

Thank you for your contribution @Tavernari 🤗

Comments were addressed :)

[Tavernari]

Thanks!! 😊