[workflows] Avoid usage of access token in issue-write.yml (llvm#94011)

This adds a new composite workflow that allows you to download artifacts
from other workflows without using an access token.

actions/download-artifact from GitHub requires an access token in order
to download artifacts from a different workflow, which is why we can't
use it here if we want to avoid using a token.

See
https://github.com/actions/download-artifact?tab=readme-ov-file#download-artifacts-from-other-workflow-runs-or-repositories

Author: tstellar

.github/workflows/issue-write.yml

@@ -24,14 +24,21 @@ jobs:
         github.event.workflow_run.conclusion == 'failure'
       )
     steps:
+      - name: Fetch Sources
+        uses: actions/checkout@v4
+        with:
+          sparse-checkout: |
+            .github/workflows/unprivileged-download-artifact/action.yml
+          sparse-checkout-cone-mode: false
       - name: 'Download artifact'
-        uses: actions/download-artifact@6b208ae046db98c579e8a3aa621ab581ff575935 # v4.1.1
+        uses: ./.github/workflows/unprivileged-download-artifact
+        id: download-artifact
         with:
-          github-token: ${{ secrets.ISSUE_WRITE_DOWNLOAD_ARTIFACT }}
           run-id: ${{ github.event.workflow_run.id }}
-          name: workflow-args
+          artifact-name: workflow-args
 
       - name: 'Comment on PR'
+        if: steps.download-artifact.outputs.artifact-id != ''
         uses: actions/github-script@v3
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
@@ -144,5 +151,7 @@ jobs:
             });
 
       - name: Dump comments file
-        if: always()
+        if: >-
+          always() &&
+          steps.download-artifact.outputs.artifact-id != ''
         run: cat comments

.github/workflows/unprivileged-download-artifact/action.yml

@@ -0,0 +1,81 @@
+name: Unprivileged Download Artifact
+description: >-
+  Download artifacts from another workflow run without using an access token.
+inputs:
+  run-id:
+    description: >-
+      The run-id for the workflow run that you want to download the artifact
+      from.  If ommitted it will download the most recently created artifact
+      from the repo with the artifact-name.
+    required: false
+  artifact-name:
+    desciption: The name of the artifact to download.
+    required: true
+
+
+outputs:
+  filename:
+    description: >-
+      The filename of the downloaded artifact or the empty string if the
+      artifact was not found.
+    value: ${{ steps.download-artifact.outputs.filename }}
+  artifact-id:
+    description: "The id of the artifact being downloaded."
+    value: ${{ steps.artifact-url.outputs.id }}
+
+
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea #v7.0.1
+      id: artifact-url
+      with:
+        script: |
+          var response;
+          if (!"${{ inputs.run-id }}") {
+            response = await github.rest.actions.listArtifactsForRepo({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              name: "${{ inputs.artifact-name }}"
+            })
+          } else {
+            response = await github.rest.actions.listWorkflowRunArtifacts({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              run_id: "${{ inputs.run-id }}",
+              name: "${{ inputs.artifact-name }}"
+            })
+          }
+
+          console.log(response)
+
+          for (artifact of response.data.artifacts) {
+            console.log(artifact);
+          }
+
+          if (response.data.artifacts.length == 0) {
+            console.log("Could not find artifact ${{ inputs.artifact-name }} for workflow run ${{ inputs.run-id }}")
+            return;
+          }
+
+          const url_response = await github.rest.actions.downloadArtifact({
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            artifact_id: response.data.artifacts[0].id,
+            archive_format: "zip"
+          })
+
+          core.setOutput("url", url_response.url);
+          core.setOutput("id", response.data.artifacts[0].id);
+
+    - shell: bash
+      if: steps.artifact-url.outputs.url != ''
+      id: download-artifact
+      run: |
+        curl -L -o ${{ inputs.artifact-name }}.zip "${{ steps.artifact-url.outputs.url }}"
+        echo "filename=${{ inputs.artifact-name }}.zip" >> $GITHUB_OUTPUT
+
+    - shell: bash
+      if: steps.download-artifact.outputs.filename != ''
+      run: |
+        unzip ${{ steps.download-artifact.outputs.filename }}