Head node proxy settings are appended to /etc/profile.d/proxy.sh on every boot

## Required Info

- AWS ParallelCluster version: 3.15.0
- OS: Ubuntu 24.04 LTS
- Region: us-east-1

## Bug description and how to reproduce

When `HeadNode/Networking/Proxy/HttpProxyAddress` is configured, AWS ParallelCluster appends proxy settings to the following file on an Ubuntu head node:

- `/etc/profile.d/proxy.sh`

The settings are not appended only once when the cluster is created. We have confirmed that new lines are added whenever the head node is stopped and started or rebooted. As a result, manual changes to `/etc/profile.d/proxy.sh` are invalidated.

## Expected behavior

- Proxy settings should be appended to `/etc/profile.d/proxy.sh` only once when the cluster is created, rather than whenever the head node is started or rebooted.
- The ParallelCluster configuration should allow `NO_PROXY` and `no_proxy` to be configured for both head nodes and compute nodes, for example by adding `HeadNode/Networking/Proxy/NoProxy`.
- Head node proxy settings should be updatable after the cluster is created.

The reported behavior affects /etc/profile.d/proxy.sh and is triggered by HeadNode/Networking/Proxy/HttpProxyAddress during head-node starts and reboots. Start by tracing that configuration entry point and the Ubuntu head-node startup path in the repository; done means repeated starts no longer duplicate settings, with NoProxy configuration and post-creation updates addressed if they remain in scope.

Written by the indexing model from the issue text.