Upgrade subosito/flutter-action in CI from v1 to v2 (#298)

Sometimes is our `ios-integration-testing` workflow failing because of:

```
Assertion failed: (timeout != -1), function uv__io_poll, file ../deps/uv/src/unix/kqueue.c, line 256.
```


![image](https://user-images.githubusercontent.com/24459435/192727315-a8dba471-f0aa-4d00-9c2e-1ed09d8cc644.png)

Therefore, I created a ticket
subosito/flutter-action#189 in the repository
of the action. The maintainer recommended me to upgrade to v2.

.github/workflows/alpha.yml

@@ -44,7 +44,7 @@ jobs:
       uses: kuhnroyal/flutter-fvm-config-action@v1
 
     - name: Install Flutter
-      uses: subosito/flutter-action@v1
+      uses: subosito/flutter-action@v2
       with:
         flutter-version: ${{ env.FLUTTER_VERSION }}
         channel: ${{ env.FLUTTER_CHANNEL }}

.github/workflows/main.yml

@@ -51,7 +51,7 @@ jobs:
       - name: Set Flutter version from FVM config file to environment variables
         uses: kuhnroyal/flutter-fvm-config-action@v1
 
-      - uses: subosito/flutter-action@v1
+      - uses: subosito/flutter-action@v2
         with:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           channel: ${{ env.FLUTTER_CHANNEL }}
@@ -83,7 +83,7 @@ jobs:
       - name: Set Flutter version from FVM config file to environment variables
         uses: kuhnroyal/flutter-fvm-config-action@v1
 
-      - uses: subosito/flutter-action@v1
+      - uses: subosito/flutter-action@v2
         with:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           channel: ${{ env.FLUTTER_CHANNEL }}
@@ -125,7 +125,7 @@ jobs:
       - name: Set Flutter version from FVM config file to environment variables
         uses: kuhnroyal/flutter-fvm-config-action@v1
 
-      - uses: subosito/flutter-action@v1
+      - uses: subosito/flutter-action@v2
         with:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           channel: ${{ env.FLUTTER_CHANNEL }}
@@ -180,7 +180,7 @@ jobs:
       - name: Set Flutter version from FVM config file to environment variables
         uses: kuhnroyal/flutter-fvm-config-action@v1
 
-      - uses: subosito/flutter-action@v1
+      - uses: subosito/flutter-action@v2
         with:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           channel: ${{ env.FLUTTER_CHANNEL }}