ci: Use GITHUB_OUTPUT envvar instead of set-output command

`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

.github/workflows/buildmusl.yaml

@@ -26,7 +26,7 @@ jobs:
         uses: actions/checkout@v2
 
       - id: tag
-        run: echo ::set-output name=TAG::${GITHUB_REF#refs/tags/}
+        run: echo "TAG=${GITHUB_REF#refs/tags/}" >> "$GITHUB_OUTPUT"
 
       - name: Install qemu dependency
         run: |

.github/workflows/lint-test.yaml

@@ -31,7 +31,7 @@ jobs:
         run: |
           changed=$(ct list-changed --config ct.yaml)
           if [[ -n "$changed" ]]; then
-            echo "::set-output name=changed::true"
+            echo "changed=true" >> "$GITHUB_OUTPUT"
           fi
 
       - name: Run chart-testing (lint)