Improvements to caching (#265)

* Bump actions/cache to v4 to suppress node warnings

* Respect `PUB_CACHE` from the environment

* Cache pub-cache separately from flutter

In order to reduce the extra work that happens when you update your
pubspec.lock, don't use `hashFiles('**/pubspec.lock')` in the hash of
the flutter installation, and separately cache the `.pubspec-cache`.

Also return the `.pub-cache` to `$HOME`, since it's not part of the tool
installation.

* Fix README example of pub-cache-path and document `PUB_CACHE`

README.md

@@ -171,11 +171,13 @@ steps:
     cache: true
     cache-key: 'flutter-:os:-:channel:-:version:-:arch:-:hash:' # optional, change this to force refresh cache
     cache-path: '${{ runner.tool_cache }}/flutter/:channel:-:version:-:arch:' # optional, change this to specify the cache path
+    pub-cache-key: 'flutter-pub:os:-:channel:-:version:-:arch:-:hash:' # optional, change this to force refresh cache of dart pub get dependencies
+    pub-cache-path: '${{ runner.tool_cache }}/flutter/:channel:-:version:-:arch:' # optional, change this to specify the cache path
     architecture: x64 # optional, x64 or arm64
 - run: flutter --version
 ```
 
-Note: `cache-key` and `cache-path` has support for several dynamic values:
+Note: `cache-key`, `pub-cache-key`, and `cache-path` has support for several dynamic values:
 
 - `:os:`
 - `:channel:`
@@ -199,5 +201,7 @@ steps:
     echo CHANNEL=${{ steps.flutter-action.outputs.CHANNEL }}
     echo VERSION=${{ steps.flutter-action.outputs.VERSION }}
     echo ARCHITECTURE=${{ steps.flutter-action.outputs.ARCHITECTURE }}
+    echo PUB-CACHE-PATH=${{ steps.flutter-action.outputs.PUB-CACHE-PATH }}
+    echo PUB-CACHE-KEY=${{ steps.flutter-action.outputs.PUB-CACHE-KEY }}
   shell: bash
 ```