Always skip update command for git based external projects

I noticed that some external projects were being reconfigured/built
with every make, even though no files in the external project had been
updated.

With some debugging I noticed that git based external projects were
re-running their "update" step every time, and that in turn caused the
configure/build/install steps to re-run as well. (Generally the build
step is a no-op as the Makefile/etc. in the external project is well
formed and doesn't do anything, but the configure/install steps still
run).

It seems related to this cmake bug:
https://gitlab.kitware.com/cmake/cmake/-/issues/19703. In short, the
git update step for an external project does not create any "done"
file that denotes that the files are still up-to-date. Without that
"done" file, the update step is always run, and that in turn causes
the other steps for the external project to re-run as well.

The best way to fix this seems to be to skip the update step by
defining an empty UPDATE_COMMAND. As long as the downloaded code for a
given hash/tag/etc does not change, the update step is unnecessary.

And if we *really* wanted to ensure unchanged dependencies, we would
download our own copies anyway.

Making this change significantly cleans up the falco build to avoid
rebuilding git based external dependencies.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

cmake/modules/DownloadStringViewLite.cmake

@@ -24,6 +24,7 @@ ExternalProject_Add(
   GIT_TAG "v1.4.0"
   CONFIGURE_COMMAND ""
   BUILD_COMMAND ""
+  UPDATE_COMMAND ""
   INSTALL_COMMAND
     ${CMAKE_COMMAND} -E copy ${STRING_VIEW_LITE_PREFIX}/src/string-view-lite/include/nonstd/string_view.hpp
     ${STRING_VIEW_LITE_INCLUDE}/nonstd/string_view.hpp)