Makefile: Make GITALY_EXECUTABLES deferred again

Recently, in b5c9c7ef (Makefile: Rename find_commands to GITALY_EXECUTABLES, 2022-03-25), we've changed the variable that holds the names of all Gitaly executable to be an immediate variable. While this is a good idea in general, it causes trouble in CI. In CI the compiled executables are put in cache, but the source files are not. So when files are pulled from cache, and any make target is built, it will expand GITALY_EXECUTABLES. Now source files are not pulled from the cache, so the cmd directory is missing. And therefore we revert it back to be a deferred variable.