Use a temp directory for CMake build

[fheinecke]

Description

CMake currently always uses ./build/cmake as a build directory when NVTE_CMAKE_BUILD_DIR is not set. This causes a couple of issues:

• Builds can collide with each other
• Generated files can become stale and require cleaning prior to every build
• Build files are not cleaned up upon success nor failure

This creates a temporary directory and cleans it up afterwards. Note: this makes it harder to inspect the build directory, because it will now be cleaned up after every run. For this use case I would recommend setting `NVTE_CMAKE_BUILD_DIR="${PWD}/build/cmake" to retain the current behavior. As a result of moving to a temp directory, object files will now not be reused between builds as well.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• 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 not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• CMake extension builds are now isolated

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[fheinecke]

/te-ci