fix: carthage script defaults to build only

[lawmicha]

Issue #, if available:
There was an error during release of 3.1.7 that can be reproduced locally:

$ bash ./build-support/carthage-build.sh archive AWSAppSync

*** xcodebuild output can be found in /var/folders/wl/yhpbkfx93g38l3k0ts5f92_1kgt2sf/T/carthage-xcodebuild.54QdZf.log
Failed to read file or folder at /Users/[USER]/awslabs/aws-mobile-appsync-sdk-ios/Cartfile.resolved: Error Domain=NSCocoaErrorDomain Code=260 "The file “Cartfile.resolved” couldn’t be opened because there is no such file." UserInfo={NSFilePath=/Users/[USER]/awslabs/aws-mobile-appsync-sdk-ios/Cartfile.resolved, NSUnderlyingError=0x7fb2a4524cb0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}

Description of changes:
It looked like the carthage workaround script has hardcoded build, and our release script ran

bash ./build-support/carthage-build.sh archive $FRAMEWORK_NAME

so it was running

carthage build archive AWSAppSync

before having to introduce the carthage workaround script, we were doing carthage archive AWSAppSync

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[palpatim]

Sorry about that! Thanks for fixing.