This PR makes it possible to run unlock_keychain with add_to_search_list: :replace without an existing default keychain.

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.
• I've added or updated relevant unit tests.

Motivation and Context

It is currently not possible to run unlock_keychain with add_to_search_list: :replace if you don't have an existing default keychain. Even when also providing set_default: true. For us this currently means that we have to first run create_keychain with default_keychain: true in order to create a temporary default dummy keychain and then unlock the actual keychain that we want to use with unlock_keychain and then finally remove the unused dummy keychain with delete_keychain.

Description

This PR wraps the read of the original/existing default keychain with a begin-rescue in order to make sure that it doesn't crash when no default keychain is set.

I can add that the reading of the original default keychain is stored in SharedValues::ORIGINAL_DEFAULT_KEYCHAIN but that it is undocumented for the unlock_keychain action (it is however documented for the create_keychain task https://docs.fastlane.tools/actions/create_keychain/#create_keychain).

I tried adding some tests for this (inside fastlane/spec/actions_specs/unlock_keychain_spec.rb) but I'm a rookie Ruby developer so I didn't manage to make it work because the test wouldn't fail even though I didn't have a default keychain set.

Finally, I can add that a lot of tests (bundle exec rspec) fail if you don't have a default keychain set. Also in master of the fastlane repo without my changes from this PR.

Testing Steps

This can be tested by removing the default keychain on your machine:

# Get default keychain
security default-keychain

# Delete default keychain
security delete keychain <default keychain from previous command>

# Validate that no default keychain is set
security default-keychain

and then run a simple lane that creates and unlocks a keychain:

lane :test do
  create_keychain(
    name: "Temporary_keychain",
    password: "abcd1234",
    default_keychain: false
  )

  unlock_keychain( # Unlock an existing keychain and add it to the keychain search list
    path: "Temporary_keychain",
    password: "abcd1234",
    add_to_search_list: :replace,
    set_default: true
  )
end

Note: To set your login keychain as the default again, run security default-keychain -s <default keychain from the first command you ran>, probably something like security default-keychain -s /Users/<your-username>/Library/Keychains/login.keychain-db.

If there is no default keychain, the output will now look like this:

...
[13:26:50]: -----------------------------
[13:26:50]: --- Step: unlock_keychain ---
[13:26:50]: -----------------------------
[13:26:50]: Reading existing default keychain 
[13:26:50]: Unlocking keychain at path: /Users/***/Library/Keychains/Temporary_keychain-db

Instead of failing with:

[13:22:02]: Called from Fastfile at line 383
[13:22:02]: ```
[13:22:02]:     381:	  )
[13:22:02]:     382:
[13:22:02]:  => 383:	  unlock_keychain( # Unlock an existing keychain and add it to the keychain search list
[13:22:02]:     384:	    path: "Temporary_keychain",
[13:22:02]:     385:	    password: "abcd1234",
[13:22:02]: ```
[13:22:02]: Shell command exited with exit status 1 instead of 0.