[macos-runner] Disable credential helper and askpass for checkout clone

17-minute hang on git clone, far past the http.lowSpeedLimit abort
threshold, isn't explained by a data-transfer stall. Now that the VM has
a real GUI session (auto-login), git-credential-osxkeychain could be
popping a GUI dialog nobody's there to dismiss, bypassing
GIT_TERMINAL_PROMPT. Disable the credential helper and force askpass to
fail immediately instead of prompting.

.gitea/workflows/beta.yml

@@ -25,9 +25,11 @@ jobs:
       - name: Checkout
         run: |
           export GIT_TERMINAL_PROMPT=0
+          export GIT_ASKPASS=/bin/false
+          export SSH_ASKPASS=/bin/false
           REPO_URL="${{ gitea.server_url }}/${{ gitea.repository }}.git"
           AUTH_URL="$(echo "$REPO_URL" | sed "s#https://#https://x-access-token:${{ gitea.token }}@#")"
-          git -c http.lowSpeedLimit=1000 -c http.lowSpeedTime=30 clone "$AUTH_URL" .
+          git -c credential.helper= -c http.lowSpeedLimit=1000 -c http.lowSpeedTime=30 clone "$AUTH_URL" .
           git checkout "${{ gitea.sha }}"
 
       - name: Set build number