Only setup *_PROXY env variables where needed (#7095)

no_proxy is a pain to get right, and having proxy variables present causes issues
(k8s components get proxy configuration after upgrade, see #7100)

It's better to only configure what require proxy:
- the runtime (containerd/docker/crio)
- the package manager + apt_key
- the download tasks

Tested with the following clusters
- 4 CentOS 8 nodes
- 1 Ubuntu 20.04 node

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

roles/bootstrap-os/tasks/bootstrap-flatcar.yml

@@ -4,7 +4,6 @@
 - name: Check if bootstrap is needed
   raw: stat /opt/bin/.bootstrapped
   register: need_bootstrap
-  environment: {}
   failed_when: false
   changed_when: false
   tags:
@@ -19,9 +18,7 @@
 - name: Run bootstrap.sh
   script: bootstrap.sh
   become: true
-  environment:
-    http_proxy: "{{ http_proxy | default('') }}"
-    https_proxy: "{{ https_proxy | default('') }}"
+  environment: "{{ proxy_env }}"
   when:
     - need_bootstrap.rc != 0