feat: support env-mapping option

[samypr100]

Ideally, we can just specify our dev drive environment variables just once rather than re-declaring them in each step.

Hence a job like this:

- uses: samypr100/setup-dev-drive@v3
- name: Step A
  env:
    CARGO_HOME: ${{ env.DEV_DRIVE }}/.cargo
    RUSTUP_HOME: ${{ env.DEV_DRIVE }}/.rustup
  run: ...
- name: Step B
  env:
    CARGO_HOME: ${{ env.DEV_DRIVE }}/.cargo
    RUSTUP_HOME: ${{ env.DEV_DRIVE }}/.rustup
  run: ...
- name: Step C
  env:
    CARGO_HOME: ${{ env.DEV_DRIVE }}/.cargo
    RUSTUP_HOME: ${{ env.DEV_DRIVE }}/.rustup
  run: ...
# ...

Becomes streamlined into something like this:

- uses: samypr100/setup-dev-drive@v3
  with:
    env-mapping: |
      CARGO_HOME,{{ DEV_DRIVE }}/.cargo
      RUSTUP_HOME,{{ DEV_DRIVE }}/.rustup
- name: Step A
  run: ...
- name: Step B
  run: ...
- name: Step C
  run: ...
# ...