Set HOME to /tekton/home for nonroot tasks

In an effort to reduce permissions for certain tasks in #1034, the HOME
directory was either set to `/home/nonroot` or was unchanged. This
caused the tasks to fail when being run under limited permissions as
they could not create `/home/nonroot` or write to `/` in some cases.

This commit follows suit with #860 where HOME is set to `/tekton/home`
so that the task can perform actions in it.

task/git-clone/0.8/git-clone.yaml

@@ -107,7 +107,7 @@ spec:
       description: |
         Absolute path to the user's home directory.
       type: string
-      default: "/home/nonroot"
+      default: "/tekton/home"
   results:
     - name: commit
       description: The precise commit SHA that was fetched by this Task.

task/kn-apply/0.2/kn-apply.yaml

@@ -33,4 +33,4 @@ spec:
       runAsUser: 65532
     env:
     - name: HOME
-      value: /home/nonroot
+      value: /tekton/home

task/kn/0.2/kn.yaml

@@ -26,6 +26,9 @@ spec:
     - "help"
   steps:
   - name: kn
+    env:
+    - name: HOME
+      value: /tekton/home
     image: "$(params.kn-image)"
     command: ["/ko-app/kn"]
     args: ["$(params.ARGS)"]

task/skopeo-copy/0.2/skopeo-copy.yaml

@@ -41,6 +41,9 @@ spec:
       default: "true"
   steps:
     - name: skopeo-copy
+      env:
+      - name: HOME
+        value: /tekton/home
       image: quay.io/skopeo/stable:v1.9.0
       script: |
         # Function to copy multiple images.

task/tkn/0.4/tkn.yaml

@@ -36,6 +36,9 @@ spec:
       default: ["--help"]
   steps:
     - name: tkn
+      env:
+      - name: HOME
+        value: /tekton/home
       image: "$(params.TKN_IMAGE)"
       script: |
         if [ "$(workspaces.kubeconfig.bound)" = "true" ] && [ -e $(workspaces.kubeconfig.path)/kubeconfig ]; then