fastlane: Couldn't automatically detect the provisioning profile mapping

New Issue Checklist

• Updated fastlane to the latest version
• I read the Contribution Guidelines
• I read docs.fastlane.tools
• I searched for existing GitHub issues

Issue Description

If the workspace contains files (at root level) that are not .xcproject, these errors are logged.

[10:01:49]: $ xcodebuild -showBuildSettings -workspace ./Studio.xcworkspace -scheme Studio\ Beta -configuration BetaRelease
[10:01:52]: Command timed out after 3 seconds on try 1 of 4, trying again with a 6 second timeout...
[10:01:56]: Couldn't automatically detect the provisioning profile mapping
[10:01:56]: Since Xcode 9 you need to provide an explicit mapping of what
[10:01:56]: provisioning profile to use for each target of your app
[10:01:56]: The plist file at path `/workspace/ios-studio/README.md/project.pbxproj` doesn't exist.
[10:01:56]: Couldn't automatically detect the provisioning profile mapping
[10:01:56]: Since Xcode 9 you need to provide an explicit mapping of what
[10:01:56]: provisioning profile to use for each target of your app
[10:01:56]: The plist file at path `/workspace/ios-studio/CHANGELOG.md/project.pbxproj` doesn't exist.
[10:01:56]: Couldn't automatically detect the provisioning profile mapping
[10:01:56]: Since Xcode 9 you need to provide an explicit mapping of what
[10:01:56]: provisioning profile to use for each target of your app
[10:01:56]: The plist file at path `/workspace/ios-studio/IAP.md/project.pbxproj` doesn't exist.

Those files should be simply ignored.

My workspace looks like this

Complete output when running fastlane, including the stack trace and command used

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Environment

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