Use a user-specific directory for tito builds

Description

Commit message says it all:

Using the same tito build directory for all users lead to permission
issues. Use instead of the common temporary location, the user-specific
temporary location.

How Has This Been Tested?

One is able to create the RPM package in her own temporary directory.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.