ExternalProject: Fix install on BUILD_ALWAYS+BUILD_BYPRODUCTS with Ninja

The `BUILD_BYPRODUCTS` option causes the ExternalProject's `build` step
in `build.ninja` to have `restat = 1`, so its "always out-of-date"
status caused by `BUILD_ALWAYS` does not propagate to the `install`
step.  Mark the latter step as explicitly always out-of-date too.

Fixes: #23820
27 jobs for !7546 with always-install-externalproject in 69 minutes and 39 seconds (queued for 3 seconds)
merge request