Allow to override update channel for build, use dev for dev builds (#4043)

Add input allowing to override the channel that's used for hassio image
downloads and in runtime for Supervisor updates, building on the option added
in #3618.

The new default is dev for dev builds, for GH releases keep using the stable
channel both for releases and pre-releases (so we could catch any stable issues
before beta is moved to stable).

To keep it DRY and idiomatic, create a new in-repo GH action for running
commands in the build container.

.github/actions/haos-builder-command/action.yml

@@ -0,0 +1,22 @@
+name: "Run command in HAOS build container"
+inputs:
+  image:
+    description: "HAOS builder image to use"
+    required: true
+  command:
+    description: "Command to run in the container"
+    required: true
+runs:
+  using: 'composite'
+  steps:
+  - name: "Run command in HAOS build container"
+    shell: bash
+    run: |
+      docker run --rm --privileged \
+        -e BUILDER_UID="$(id -u)" \
+        -e BUILDER_GID="$(id -g)" \
+        -v "${GITHUB_WORKSPACE}:/build" \
+        -v "/mnt/cache:/cache" \
+        -v "/mnt/output:/build/output" \
+        ${{ inputs.image }} \
+        ${{ inputs.command }}