feat: add support for caching

```yaml
  - uses: cashapp/activate-hermit@v1
    with:
      cache: true
```

This necessitated switching to a composite action, and I couldn't figure
out a way to keep the existing JS action when using it. TBH I think the
script approach is a bit neater anyway.

Also introduced a CI test for the action, and verified it with `act`.

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ./
+        with:
+          cache: true

Justfile

@@ -0,0 +1,3 @@
+# Test the action using act
+test:
+  act --container-architecture linux/amd64

action.yml

@@ -5,6 +5,26 @@ inputs:
     description: 'The directory with the Hermit environment (contains `bin/hermit`)'
     required: false
     default: './'
+  cache:
+    description: 'If "true", installed Hermit packages will be cached.'
+    required: false
+    default: 'false'
 runs:
-  using: 'node16'
-  main: 'dist/index.js'
+  using: composite
+  steps:
+    - if: ${{ inputs.cache == 'true' }}
+      uses: theowenyoung/folder-hash@v3
+      id: bin-hash
+      working-directory: ${{ inputs.working-directory }}
+      with:
+        path: ./bin
+    - if: ${{ inputs.cache == 'true' }}
+      uses: actions/cache@v3
+      with:
+        path: ${{ runner.os == 'macOS' && '~/Library/Caches/hermit/pkg' || '~/.cache/hermit/pkg' }}
+        key: ${{ runner.os }}-hermit-cache-${{ steps.bin-hash.outputs.hash }}
+        restore-keys: |
+          ${{ runner.os }}-hermit-cache-
+    - shell: bash
+      working-directory: ${{ inputs.working-directory }}
+      run: ./bin/hermit env --raw >> "$GITHUB_ENV"

bin/.just-1.16.0.pkg

@@ -0,0 +1 @@
+hermit

bin/act

@@ -0,0 +1 @@
+.act-0.2.56.pkg

bin/just

@@ -0,0 +1 @@
+.just-1.16.0.pkg