Update architecture default as runner.arch (#153)

* Update architecture default as `runner.arch`

* Update to convert `ARCH` as lowercase

* Refactor for `ARCH` variable

action.yml

@@ -27,7 +27,7 @@ inputs:
   architecture:
     description: 'The architecture of Flutter SDK executable (x64 or arm64)'
     required: false
-    default: 'x64'
+    default: ${{ runner.arch }}
 runs:
   using: 'composite'
   steps:

setup.sh

@@ -95,7 +95,7 @@ done
 
 CHANNEL="${@:$OPTIND:1}"
 VERSION="${@:$OPTIND+1:1}"
-ARCH="${@:$OPTIND+2:1}"
+ARCH=$(echo "${@:$OPTIND+2:1}" | awk '{print tolower($0)}')
 
 SDK_CACHE="$(transform_path ${CACHE_PATH})"
 PUB_CACHE="$(transform_path ${CACHE_PATH}/.pub-cache)"