feat(ci): add pip caching for faster build times

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.74%. Comparing base (e349fa4) to head (c887b78).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11 +/- ## ======================================= Coverage 95.74% 95.74% ======================================= Files 3 3 Lines 282 282 ======================================= Hits 270 270 Misses 12 12 ``` | [Flag](https://app.codecov.io/gh/omsf-eco-infra/gha-runner/pull/11/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omsf-eco-infra) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/omsf-eco-infra/gha-runner/pull/11/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omsf-eco-infra) | `95.74% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=omsf-eco-infra#carryforward-flags-in-the-pull-request-comment) to find out more.

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[dwhswenson]

(With the optional change, either make the change and merge, or merge without change -- passing the baton back to you)