chore(ci): use environment files to add QEMU to PATH

<https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/>

> A moderate security vulnerability has been identified in the GitHub
> Actions runner that can allow environment variable and path injection
> in workflows that log untrusted data to STDOUT. This can result in
> environment variables being introduced or modified without the
> intention of the workflow author.
>
> [...]
>
> Action and workflow authors who are setting environment variables via
> STDOUT should update any usage of the `set-env` and `add-path`
> workflow commands to use the new environment files.

Author: yvt

.github/actions/install-qemu/action.yml

@@ -16,7 +16,7 @@ runs:
         qemu_timestamp=$HOME/.qemu/${{ inputs.version }}
 
         # Add the prefix to path
-        echo '::add-path::'$qemu_prefix/bin
+        echo "$qemu_prefix/bin" >> $GITHUB_PATH
 
         # Skip the remaining steps if QEMU has already been built
         if [ -f "$qemu_timestamp" ]; then