Fix installers for kustomize >= 3.6.0

[stefanprodan]

The current installers are broken for kustomize >= 3.6.0:

$ kustomize build github.com/weaveworks/flagger//kustomize/kubernetes?ref=v1.0.0 | kubectl apply -f -

Error: accumulating resources: accumulateFile "accumulating resources from '../base/flagger/': evalsymlink failure on '/private/var/folders/77/3y6x_p2j2g9fspdkzjbm5_s40000gn/T/kustomize-948385415/base/flagger' : lstat /private/var/folders/77/3y6x_p2j2g9fspdkzjbm5_s40000gn/T/kustomize-948385415/base: no such file or directory", loader.New "Error loading ../base/flagger/ with git: url lacks host: ../base/flagger/, dir: evalsymlink failure on '/private/var/folders/77/3y6x_p2j2g9fspdkzjbm5_s40000gn/T/kustomize-948385415/base/flagger' : lstat /private/var/folders/77/3y6x_p2j2g9fspdkzjbm5_s40000gn/T/kustomize-948385415/base: no such file or directory, get: invalid source string: ../base/flagger/"

This PR adds the https:// prefix to the base URL and removes the //:

 $ kustomize build https://github.com/weaveworks/flagger/kustomize/kubernetes?ref=v1.0.0 | kubectl apply -f -

namespace/flagger-system created
customresourcedefinition.apiextensions.k8s.io/alertproviders.flagger.app configured
customresourcedefinition.apiextensions.k8s.io/canaries.flagger.app configured
customresourcedefinition.apiextensions.k8s.io/metrictemplates.flagger.app configured
serviceaccount/flagger created
serviceaccount/flagger-prometheus created
clusterrole.rbac.authorization.k8s.io/flagger configured
clusterrole.rbac.authorization.k8s.io/flagger-prometheus created
clusterrolebinding.rbac.authorization.k8s.io/flagger configured
clusterrolebinding.rbac.authorization.k8s.io/flagger-prometheus created
configmap/flagger-prometheus-5hdhmkhck9 created
service/flagger-prometheus created
deployment.apps/flagger created
deployment.apps/flagger-prometheus created

[mathetake]

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