Use stable v5.6 version of swiftwasm-action (TokamakUI#477)

After `carton` 0.14.0 was tagged, I've updated `swiftwasm-action` to use that latest release. With that, corresponding `v5.6` was tagged, which I propose to use in `ci.yml` instead of the `main` branch.

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - uses: swiftwasm/swiftwasm-action@main
+      - uses: swiftwasm/swiftwasm-action@v5.6
         with:
           shell-action: carton bundle --product TokamakDemo
 
@@ -20,7 +20,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - uses: swiftwasm/swiftwasm-action@main
+      - uses: swiftwasm/swiftwasm-action@v5.6
         with:
           shell-action: carton test
 
@@ -83,7 +83,7 @@ jobs:
   gtk_ubuntu_18_04_build:
     runs-on: ubuntu-latest
     container:
-     image: swiftlang/swift:nightly-bionic
+      image: swiftlang/swift:nightly-bionic
 
     steps:
       - uses: actions/checkout@v2
@@ -98,7 +98,7 @@ jobs:
   gtk_ubuntu_20_04_build:
     runs-on: ubuntu-latest
     container:
-     image: swiftlang/swift:nightly-focal
+      image: swiftlang/swift:nightly-focal
 
     steps:
       - uses: actions/checkout@v2