ci: Cache .cache/pip instead of .venv

[fischeti]

• Merges init-deps and init-pd CI jobs, which essentially did the same thing.
• Caching the entire virtual environment causes some spurious issues in the CI. Instead, only the pip packages are now cached, similar as in the snitch cluster.

[Copilot]

Pull Request Overview

This PR updates the CI caching strategy to cache only pip packages instead of the entire virtual environment and consolidates the init-deps and init-pd jobs into a single job.

• Updated the Makefile to use a new PIP_CACHE_DIR variable for pip package installation.
• Modified .gitlab/common.yml to cache only the pip packages and removed caching of the virtual environment.
• Updated .gitlab-ci.yml to replace references to the removed init-pd job with init-deps and added artifacts for pd/ci.yml.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Makefile	Introduced PIP_CACHE_DIR and updated pip install commands to use caching.
.gitlab/common.yml	Replaced caching of .venv with .cache/pip and centralized cache settings.
.gitlab-ci.yml	Updated job dependencies and artifacts, replacing the init-pd reference.

Comments suppressed due to low confidence (1)

.gitlab/common.yml:6

• [nitpick] Verify that the use of the YAML merge key (<<: *global_cache) is supported by the GitLab runner version in use and is clearly documented.

PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"

[Copilot]

[nitpick] Consider adding a brief comment explaining the purpose of PIP_CACHE_DIR to improve clarity for future maintainers.

[Copilot]

Ensure that all downstream jobs and documentation reflect the change from init-pd to init-deps to avoid potential job dependency issues.

[colluca]

LGTM