Since commit v3.13.0-rc1~39^2 (add_custom_{command,target}:
WORKING_DIRECTORY generator expressions, 2018-09-22) the
`WORKING_DIRECTORY` option accepts generator expressions.
Fix support for the case of a leading generator expression
by deferring conversion to an absolute path until after
evaluation of the generator expression.
Fixes: #18543
When running an in-source build the CustomCommandWorkingDirectory test
created a copy of a source file in the same directory it was running on.
This breaks when byproducts are cleaned (e.g. via Ninja) because it
deletes one of the source files.
This was inadvertently testing the assumed sources behavior in Ninja (no
other test seems to exercise it). There is now a test explicitly testing
it in `RunCMake.Ninja`, so fix this test to work properly regardless of
the assumed sources behavior.