Fix validation errors in timeline_semaphores sample

[gary-sweet]

Description

The command buffer was being implicitly reset, but wasn't resettable.

General Checklist:

Please ensure the following points are checked:

• [x] My code follows the coding style
• [x] I have reviewed file licenses
• [x] I have commented any added functions (in line with Doxygen)
• [x] I have commented any code that could be hard to understand
• [x] My changes do not add any new compiler warnings
• [x] My changes do not add any new validation layer errors or warnings
• [x] I have used existing framework/helper functions where possible
• [x] My changes do not add any regressions
• [x] I have tested every sample to ensure everything runs correctly
• [x] This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:

• [x] I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
• [x] My changes build on Linux

[asuessenbach]

Would you mind to resolve this issue like it's done in #833?

[gary-sweet]

Would you mind to resolve this issue like it's done in #833?

Done. That's a much nicer solution, thanks.

[SaschaWillems]

Code changes look good to me, sadly can't test due to #588