CI(label-for-external-users): add retry logic for unexpected errors (#8938)

## Problem

One of the PRs opened by a `neondatabase` org member got labelled as
`external` because the `gh api` call failed in the wrong way:
```
Get "https://api.github.com/orgs/neondatabase/members/<username>": dial tcp 140.82.114.5:443: i/o timeout
is-member=false
```

## Summary of changes
- Check that the error message is expected before labelling PRs
- Retry `gh api` call for 10 times in case of unexpected error messages
- Add `workflow_dispatch` trigger

.github/workflows/label-for-external-users.yml

@@ -7,6 +7,11 @@ on:
   pull_request_target:
     types:
       - opened
+  workflow_dispatch:
+    inputs:
+      github-actor:
+        description: 'GitHub username. If empty, the username of the current user will be used'
+        required: false
 
 # No permission for GITHUB_TOKEN by default; the **minimal required** set of permissions should be granted in each job.
 permissions: {}
@@ -26,12 +31,31 @@ jobs:
       id: check-user
       env:
         GH_TOKEN: ${{ secrets.CI_ACCESS_TOKEN }}
+        ACTOR: ${{ inputs.github-actor || github.actor }}
       run: |
-        if gh api -H "Accept: application/vnd.github+json" -H "X-GitHub-Api-Version: 2022-11-28" "/orgs/${GITHUB_REPOSITORY_OWNER}/members/${GITHUB_ACTOR}"; then
-          is_member=true
-        else
-          is_member=false
-        fi
+        expected_error="User does not exist or is not a member of the organization"
+        output_file=output.txt
+
+        for i in $(seq 1 10); do
+          if gh api "/orgs/${GITHUB_REPOSITORY_OWNER}/members/${ACTOR}" \
+              -H "Accept: application/vnd.github+json" \
+              -H "X-GitHub-Api-Version: 2022-11-28" > ${output_file}; then
+
+            is_member=true
+            break
+          elif grep -q "${expected_error}" ${output_file}; then
+            is_member=false
+            break
+          elif [ $i -eq 10 ]; then
+            title="Failed to get memmbership status for ${ACTOR}"
+            message="The latest GitHub API error message: '$(cat ${output_file})'"
+            echo "::error file=.github/workflows/label-for-external-users.yml,title=${title}::${message}"
+
+            exit 1
+          fi
+
+          sleep 1
+        done
 
         echo "is-member=${is_member}" | tee -a ${GITHUB_OUTPUT}