Use vmtest-action version 0.8 in CI

With vmtest-action version 0.8 released, it makes the most sense for us
to consume this version as opposed to the current snapshot from the
master branch. This way, we anticipate to consume stable releases and
have Dependabot manage version bumps for us.

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

.github/workflows/test.yml

@@ -208,7 +208,7 @@ jobs:
         EOF
         chmod a+x main.sh
     - name: Test and gather coverage
-      uses: danobi/vmtest-action@master
+      uses: danobi/vmtest-action@v0.8
       with:
         kernel: bzImage
         command: sh -c 'cd ${{ github.workspace }} && ./main.sh'