ExternalProject: Prevent COMMAND from being treated as a true keyword
The known keywords for each function are obtained by scraping the documentation for lines matching a particular regular expression. In !1037 (merged), the docs were overhauled and the COMMAND docs subsequently matched the regular expression when they shouldn't have. This made COMMAND appear as a true keyword, which thwarted the special handling logic elsewhere for the intended use of COMMAND arguments.
Fixes: #17198 (closed)