gh/vmtests: truncate artifacts if run was successful

There does not seem to be an easy way to delete artifacts of the
existing run (see details below). Instead, this patch truncates the
build artifact so at least we do not waste space.

I tried the following:
- name: Delete artifacts
uses: actions/github-script@v3
with:
  github-token: ${{ github.token }}
  script: |
    const {GITHUB_RUN_ID, GITHUB_REPOSITORY} = process.env
    const artifactsURL = `/repos/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}/artifacts`
    console.log(`Deleting artifacs from ${artifactsURL}`)
    const response = await github.request(`GET ${artifactsURL}`)
    console.log(response.data)
    for (artifact of response.data.artifacts) {
	  console.log(`Deleting artifact with name=${artifact.name}, id=${artifact.id}`)
	  const deleteResponse = await github.request(`DELETE /repos/${repo}/actions/artifacts/${artifact.id}`)
    }

But it did not work, because the GITHUB_RUN_ID does not seem to be available over the REST API until
after the action is finished.

The logs from my action printed:
Deleting artifacs from /repos/cilium/tetragon/actions/runs/2817658181/artifacts
...
{ total_count: 0, artifacts: [] }

But if I curl the above URL after the action, I get:
{
  "total_count": 1,
  "artifacts": [
    {
      "id": 322874447,
      "node_id": "MDg6QXJ0aWZhY3QzMjI4NzQ0NDc=",
      "name": "tetragon-build",
      "size_in_bytes": 6,
      "url": "https://github.com/gitapi/repos/cilium/tetragon/actions/artifacts/322874447",
      "archive_download_url": "https://github.com/gitapi/repos/cilium/tetragon/actions/artifacts/322874447/zip",
      "expired": false,
      "created_at": "2022-08-08T11:50:54Z",
      "updated_at": "2022-08-08T11:50:54Z",
      "expires_at": "2022-08-13T11:49:30Z",
      "workflow_run": {
        "id": 2817658181,
        "repository_id": 473137633,
        "head_repository_id": 473137633,
        "head_branch": "pr/kkourt/lvhtest",
        "head_sha": "ea22990af2283c38aac92f7e6febacabf3786280"
      }
    }
  ]
}

For (even) more context, see: actions/upload-artifact#5

Signed-off-by: Kornilios Kourtis <kornilios@isovalent.com>

.github/workflows/vmtests.yml

@@ -127,3 +127,17 @@ jobs:
         name: tetragon-vmtests-${{ matrix.kernel }}-${{ matrix.group }}-results
         path: go/src/github.com/cilium/tetragon/tests/vmtests/vmtests-results-*
         retention-days: 5
+  post-test:
+    runs-on: ubuntu-latest
+    needs: [test]
+    if: success()
+    steps:
+    - name: Create truncated build file
+      run: |
+        touch /tmp/tetragon.tar
+    - name: Upload truncated file
+      uses: actions/upload-artifact@v3
+      with:
+         name: tetragon-build
+         path: /tmp/tetragon.tar
+         retention-days: 1