Use correct variable name for Python path in AWS auth tests. (#1019)

Author: matthewdale

.evergreen/config.yml

@@ -2119,21 +2119,21 @@ axes:
           GCC_PATH: "/cygdrive/c/ProgramData/chocolatey/lib/mingw/tools/install/mingw64/bin"
           GO_DIST: "C:\\golang\\go1.17"
           SKIP_ECS_AUTH_TEST: true
-          PYTHON3: "C:/python/Python38/python.exe"
+          PYTHON3_BINARY: "C:/python/Python38/python.exe"
       - id: "ubuntu1804-64-go-1-17"
         display_name: "Ubuntu 18.04"
         run_on: ubuntu1804-test
         variables:
           GO_DIST: "/opt/golang/go1.17"
-          PYTHON3: python3
+          PYTHON3_BINARY: "/opt/python/3.8/bin/python3"
       - id: "osx-go-1-17"
         display_name: "MacOS 10.15"
         run_on: macos-1015
         variables:
           GO_DIST: "/opt/golang/go1.17"
           SKIP_ECS_AUTH_TEST: true
           SKIP_EC2_AUTH_TEST: true
-          PYTHON3: python3
+          PYTHON3_BINARY: python3
 
 task_groups:
   - name: serverless_task_group