website gha.md: job env variable sets LIMA_VERSION

[msgilligan]

1. Fixes the cache action which was resolving ${{ env.LIMA_VERSION }} to an empty string.

2. Makes the build more functional/reproducible by not getting LIMA_VERSION from an external source.

[msgilligan]

I have made and tested the above change here:
nixos-lima/nixos-lima-config-sample@2e5bd7c

You can see the output from the job before the change here:

Run actions/cache@v4
  with:
    path: ~/.cache/lima
    key: lima-
    enableCrossOsArchive: false
    fail-on-cache-miss: false
    lookup-only: false
    save-always: false
Cache not found for input keys: lima-

And after the change here:

Run actions/cache@v4
  with:
    path: ~/.cache/lima
    key: lima-v1.0.4
    enableCrossOsArchive: false
    fail-on-cache-miss: false
    lookup-only: false
    save-always: false
  env:
    LIMA_VERSION: v1.0.4
Cache not found for input keys: lima-v1.0.4

Here is the successful job:
https://github.com/nixos-lima/nixos-lima-config-sample/actions/runs/12997768752/job/36249456914

[msgilligan]

@AkihiroSuda And here's an updated, passing, test job: https://github.com/nixos-lima/nixos-lima-config-sample/actions/runs/13019266424/job/36315933324

[AkihiroSuda]

Thanks