Update mplfinance_checks.yml

same as previous.  two spots to use ubuntu-20.04 instead of ubuntu-latest.

Author: DanielGoldfarb

.github/workflows/mplfinance_checks.yml

@@ -2,7 +2,7 @@ name: mplfinance Checks
 on: [ workflow_dispatch, pull_request ]
 jobs:
   Regression_Tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8, 3.9]