Log Message:
regress/show-all: demonstrate wrong output of show-all

A real-life example of this pattern is EXTRACT_CMD_DEFAULT, which refers
to DOWNLOADED_DISTFILE, which is defined as "$${extract_file}".

Until mk/extract/extract.mk r1.40, that argument was discarded from the
output completely. With the surrounding quotes at least the quotes are
visible in the output of "bmake show-all-extract".