specify shell and pem extensin

Signed-off-by: Shalier Xia <shalierxia@microsoft.com>

.github/workflows/automerge.yml

@@ -21,13 +21,14 @@ jobs:
       env:
         ID: ${{secrets.APP_ID}}
         PK: ${{secrets.APP_PRIVATE_KEY}}
+      shell: bash
       run: |
         issueTime=$(date +%s)
         expireTime=$(date -d "$expireTime + 600 seconds" +%s)
         header=$(echo '{ "alg": "RS256", "typ": "JWT" }' | jq -r '(. | @base64)')
         payload=$(echo '{"iss": '"$ID"',"iat": '$issueTime' ,"exp": '$expireTime'}'| jq -r '(. | @base64)'| sed s/\+/-/ | sed -E s/=+$//)
-        echo "$PK" > key
-        signature=$(echo -n "$header.$payload" | openssl dgst -sha256 -binary -sign key | openssl enc -base64 | tr -d '\n=' | tr -- '+/' '-_')
+        echo "$PK" > key.pem
+        signature=$(echo -n "$header.$payload" | openssl dgst -sha256 -binary -sign key.pem | openssl enc -base64 | tr -d '\n=' | tr -- '+/' '-_')
         tokenURL=$(curl -i -s -X GET -H "Authorization: Bearer $header.$payload.$signature" -H "Accept: application/vnd.github.v3+json" https://api.github.com/app/installations | grep 'access_tokens_url' | awk '{print $2}' | sed -e 's/^"//' -e 's/",$//')
         token=$(curl -i -s -X POST -H "Authorization: Bearer $header.$payload.$signature" -H "Accept: application/vnd.github.v3+json" "$tokenURL" | grep 'token'| awk '{print $2}' | sed -e 's/^"//' -e 's/",$//')
         echo "GITHUB_TOKEN=$token" >> "$GITHUB_ENV"