Use gopkg.in/yaml.v2 for Helmfile v0.x (#609)

This should fix #435 for Helmfile v0.x releases since the next v0.150.0.
We introduce a new envvar to opt-in to the new YAML library, so that you can give it a shot before upgrading your Helmfile to v1. The same envvar can be used to opt-out of the new YAML library after you upgrade to Helmfile v1, giving you a more flexible migration story.

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

.github/workflows/ci.yaml

@@ -61,6 +61,7 @@ jobs:
            plugin-secrets-version: 3.15.0
            plugin-diff-version: 3.5.0
            extra-helmfile-flags:
+           v1mode:
          - helm-version: v3.9.4
            kustomize-version: v4.5.7
            # We assume that the helm-secrets plugin is supposed to
@@ -71,16 +72,26 @@ jobs:
            plugin-secrets-version: 4.0.0
            plugin-diff-version: 3.6.0
            extra-helmfile-flags:
+           v1mode:
          - helm-version: v3.10.3
            kustomize-version: v4.4.1
            plugin-secrets-version: 3.15.0
            plugin-diff-version: 3.5.0
            extra-helmfile-flags:
+           v1mode:
          - helm-version: v3.10.3
            kustomize-version: v4.5.7
            plugin-secrets-version: 4.0.0
            plugin-diff-version: 3.6.0
            extra-helmfile-flags:
+           v1mode:
+         # Helmfile v1
+         - helm-version: v3.10.3
+           kustomize-version: v4.5.7
+           plugin-secrets-version: 4.0.0
+           plugin-diff-version: 3.6.0
+           extra-helmfile-flags:
+           v1mode: "true"
          # In case you need to test some optional helmfile features,
          # enable it via extra-helmfile-flags below.
          - helm-version: v3.10.3
@@ -88,6 +99,7 @@ jobs:
            plugin-secrets-version: 4.0.0
            plugin-diff-version: 3.6.0
            extra-helmfile-flags: "--enable-live-output"
+           v1mode:
     steps:
     - uses: actions/checkout@v2
     - name: Cache libraries
@@ -131,6 +143,7 @@ jobs:
         HELMFILE_HELM3: 1
         TERM: xterm
         EXTRA_HELMFILE_FLAGS: ${{ matrix.extra-helmfile-flags }}
+        HELMFILE_V1MODE: ${{ matrix.v1mode }}
       run: make integration
   e2e_tests:
     needs: tests