Use signing keychain for sign_app.sh

[mbacchi]

yarn run create_dist has been failing with keychain errors
recently, this corrects the keychain file name.

Partial fix for brave/brave-browser#387

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Needed or QA/No-QA-Needed) to include the closed issue in milestone

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[RyanJarv]

Few comments

[RyanJarv]

This should be the name of the keychain, login is usually the default so that makes sense here still I think.

[RyanJarv]

"/Library/Keychains/login.${mac_signing_keychain}-db"

should be imo

"/Library/Keychains/${mac_signing_keychain}.keychain-db"

The keychain-db is always the same and doesn't change, the first part is the keychain name which generally defaults to login

[RyanJarv]

lgtm