Use a file to record successful manifests build

Using the directory cmd/flux/manifests as a prerequisite causes a
problem: if the script that creates the files within fails, the next
invocation of make will see the directory and assume it
succeeded. Since the executable expects certain files to be present,
but they are not explicit prerequisites of the recipe for building the
binary, this results in a successful build but a broken `flux`
executable.

Instead, depend on a file that's explicitly updated when the script
has succeeded, and which itself depends on the inputs.

A couple of the CI workflows run

    make cmd/flux/manifests

before doing other things, presumably as a way to avoid running the
whole test suite in a CI pipeline for some purpose other than testing,
so these needed changing as well.

Signed-off-by: Michael Bridgen <michael@weave.works>

.gitignore

@@ -19,6 +19,7 @@ dist/
 bin/
 output/
 cmd/flux/manifests/
+cmd/flux/.manifests.done
 
 # Docs
 site/