Replace in evergreen.yml

In the WiredTiger evergreen.yml configuration file, currently the per-task working_dir variable is set to different values (e.g. "wiredtiger/build_posix", 'wiredtiger_build_posix/test/csuite") for different tasks depending on the test type. It would be great to pursue a consistent setting e.g. to "wiredtiger/build_posix", to provide a consistent value across tasks and a reliable value of working_dir that build variant level variables could refer to.