Make manifests dir bundle.sh configurable

There was an assumption in this script that it is always executed in Git
repository/directory, this is however not always true, for example when
one downloads the `.tar.gz` that is made available for every release
by GitHub (and used in one of our AUR packages).

This commit changes this, and makes the first argument of `bundle.sh`
configurable, so a custom manifests directory can always be defined
_without_ relying on Git.

Omitting it, or passing an empty string, will still fall back to the
previous behavior of using `git rev-parse --show-toplevel`.

Signed-off-by: Hidde Beydals <hello@hidde.co>

.github/workflows/release.yaml

@@ -31,7 +31,7 @@ jobs:
       - name: Generate manifests
         run: |
           make cmd/flux/manifests
-          ./manifests/scripts/bundle.sh ./output manifests.tar.gz
+          ./manifests/scripts/bundle.sh "" ./output manifests.tar.gz
           kustomize build ./manifests/install > ./output/install.yaml
       - name: Run GoReleaser
         uses: goreleaser/goreleaser-action@v1